Guided wave devices with selectively thinned piezoelectric layers

ABSTRACT

A micro-electrical-mechanical system (MEMS) guided wave device includes a plurality of electrodes arranged below a piezoelectric layer (e.g., either embedded in a slow wave propagation layer or supported by a suspended portion of the piezoelectric layer) and configured for transduction of a lateral acoustic wave in the piezoelectric layer. The piezoelectric layer permits one or more additions or modifications to be made thereto, such as trimming (thinning) of selective areas, addition of loading materials, sandwiching of piezoelectric layer regions between electrodes to yield capacitive elements or non-linear elastic convolvers, addition of sensing materials, and addition of functional layers providing mixed domain signal processing utility.

STATEMENT OF RELATED APPLICATIONS

This application is a non-provisional of U.S. Provisional PatentApplication No. 62/281,805 filed on Jan. 22, 2016. The entire contentsof the foregoing application are hereby incorporated by reference as ifset forth fully herein.

TECHNICAL FIELD

The present disclosure relates to electromechanical components utilizingacoustic wave propagation in piezoelectric layers, and in particular tothin film guided wave structures and methods for making such structures.Such structures may be used, for example, in radio frequencytransmission circuits, sensor systems, signal processing systems, andthe like.

BACKGROUND

Micro-electrical-mechanical system (MEMS) devices come in a variety oftypes and are utilized across a broad range of applications. One type ofMEMS device that may be used in applications such as radio frequency(RF) circuitry is a MEMS vibrating device (also known as a resonator). AMEMS resonator generally includes a vibrating body in which apiezoelectric layer is in contact with one or more conductive layers.Piezoelectric materials acquire a charge when compressed, twisted, ordistorted. This property provides a transducer effect between electricaland mechanical oscillations or vibrations. In a MEMS resonator, anacoustic wave may be excited in a piezoelectric layer in the presence ofan alternating electrical signal, or propagation of an elastic wave in apiezoelectric material may lead to generation of an electrical signal.Changes in the electrical characteristics of the piezoelectric layer maybe utilized by circuitry connected to a MEMS resonator device to performone or more functions.

Guided wave resonators include MEMS resonator devices in which anacoustic wave is confined in part of a structure, such as in thepiezoelectric layer. Confinement may be provided on at least onesurface, such as by reflection at a solid/air interface, or by way of anacoustic mirror (e.g., a stack of layers referred to as a Bragg mirror)capable of reflecting acoustic waves. Such confinement may significantlyreduce or avoid dissipation of acoustic radiation in a substrate orother carrier structure.

Various types of MEMS resonator devices are known, including devicesincorporating interdigital transducer (IDT) electrodes and periodicallypoled transducers (PPTs) for lateral excitation. Examples of suchdevices are disclosed in U.S. Pat. No. 7,586,239 and U.S. Pat. No.7,898,158 assigned to RF Micro Devices, Inc. (Greensboro, N.C., USA),wherein the contents of the foregoing patents are hereby incorporated byreference herein. Devices of these types are structurally similar tofilm bulk acoustic resonator (FBAR) devices, in that they each embody asuspended piezoelectric membrane. Suspended piezoelectric membranedevices, and particularly IDT-type membrane devices, are subject tolimitations of finger resistivity and power handling due to poor thermalconduction in the structures. Additionally, IDT-type and PPT-typemembrane devices may require stringent encapsulation, such as hermeticpackaging with a near-vacuum environment.

Plate wave (also known as lamb wave) resonator devices are also known,such as described in U.S. Patent Application Publication No.2010-0327995 A1 to Reinhardt et al. (“Reinhardt”). Compared to surfaceacoustic wave (SAW) devices, plate wave resonators may be fabricatedatop silicon or other substrates and may be more easily integrated intoradio frequency circuits. Reinhardt discloses a multi-frequency platewave type resonator device including a silicon substrate, a stack ofdeposited layers (e.g., SiOC, SiN, SiO₂, and Mo) constituting a Braggmirror, a deposited AlN piezoelectric layer, and a SiN passivationlayer. At least one resonator includes a differentiation layerunderlying a piezoelectric layer and arranged to modify the couplingcoefficient of the resonator so as to have a determined usefulbandwidth.

A representative MEMS guided wave device 10 of a conventional type knownin the art is shown in FIG. 1. The device 10 includes a piezoelectriclayer 12 arranged over a substrate 14, with top side electrodes in theform of an IDT 18 bounded laterally on two ends by a pair of reflectorgratings 20. The IDT 18 includes electrodes with a first conductingsection and a second conducting section that are inter-digitallydispersed on a top surface of the piezoelectric layer 12. The IDT 18 andthe reflector gratings 20 include a number of fingers 24 that areconnected to respective bus bars 22. For the reflector gratings 20, allfingers 24 connect to each bus bar 22. For the IDT 18, alternatingfingers 24 connect to different bus bars 22, as depicted. (Actualreflector gratings 20 and IDT 18 may include larger numbers of fingers24 than illustrated.) For the IDT 18, the fingers 24 are parallel to oneanother and aligned in an acoustic region that encompasses the area inwhich the IDT 18 and its corresponding reflector gratings 20 reside. Atleast one wave is generated when the IDT 18 is excited with electricalsignals (e.g., supplied via bus bars 22). Acoustic waves essentiallytravel perpendicular to the length of the fingers 24 and essentiallyreside in the acoustic region encompassing the area including the IDT 18and the reflector gratings 20. The operating frequency of the MEMSguided wave device 10 is a function of the pitch (P) representing thespacing between fingers 24 of the respective IDT 18. The wavelength λ ofan acoustic wave transduced by the IDT 18 equals two times the pitch orseparation distance between adjacent electrodes (fingers 24) of oppositepolarity, and the wavelength λ also equals the separation distancebetween closest electrodes (e.g., fingers 24) of the same polarity.

In any of the above-described devices, access to exposed portions of anactive region of a piezoelectric layer is limited, since an activeregion is typically obscured by presence of electrodes such as IDTs.

Additionally, it may be difficult to adjust one or more properties of aguided wave device, such as frequency, coupling coefficient, temperaturecompensation characteristics, velocity, phase, capacitance, orpropagative wave mode, over portions or an entirety of a guided wavedevice. It may also be difficult to integrate one or more functionalstructures with a guided wave device without interfering with placementof electrodes such as IDTs.

Accordingly, there is a need for guided wave devices that can beefficiently manufactured, and that enable production of devices withenhanced utility.

SUMMARY

The present disclosure provides a micro-electrical-mechanical system(MEMS) guided wave device in which a plurality of electrodes is arrangedbelow a piezoelectric layer and configured for transduction of a lateralacoustic wave in the piezoelectric layer. In certain implementations,the plurality of electrodes is embedded in a slow wave propagation layerarranged below the piezoelectric layer. In certain implementations, thepiezoelectric layer embodies a single crystal piezoelectric material. Incertain implementations, at least one guided wave confinement structure,such as a fast wave propagation material and/or a Bragg mirror, isarranged proximate to (e.g., below) the slow wave propagation layer andis configured to confine a lateral acoustic wave in the piezoelectriclayer and the slow wave propagation layer. In certain implementations,at least one guided wave confinement structure includes a cavity orrecess arranged below a piezoelectric layer, such that a portion of thepiezoelectric layer is suspended. By arranging electrodes below thepiezoelectric layer, a surface of the piezoelectric layer is availableto permit one or more additions or modifications to be made to thepiezoelectric layer, thereby enabling production of devices withenhanced utility.

Various additions or modifications to a piezoelectric layer may be made.In certain implementations, selective areas of piezoelectric layers maybe trimmed (e.g., by ion milling and/or etching) to provide differentthickness regions, thereby permitting adjustment of frequency and/orcoupling strength, and enabling formation of multi-frequency devices(e.g., when electrodes of different periodicities are provided). Incertain implementations, one or more loading materials are arranged incontact with one or more portions of a piezoelectric layer to locallyalter a property of a lateral acoustic wave transduced in thepiezoelectric layer, such as frequency, coupling coefficient,temperature compensation characteristics, velocity, capacitance, orpropagative wave mode. In certain implementations, one or more regionsof a piezoelectric layer are sandwiched between one or more embeddedelectrodes and one or more top side electrodes to yield one or morecapacitive elements, such as may be optionally arranged in series orarranged in parallel with at least some embedded electrodes that areconfigured for transduction of a lateral acoustic wave in thepiezoelectric layer. In certain implementations, at least one sensingmaterial is arranged over at least a portion of the piezoelectric layer,wherein at least one property of the at least one sensing material isconfigured to change in exposure to an environment proximate to the atleast one sensing material, and at least one wave propagation propertyof the piezoelectric layer may be altered in response to such change. Incertain implementations, at least one functional layer is arranged on orover at least a portion of the piezoelectric layer, and configured tointeract with the piezoelectric layer to provide mixed domain signalprocessing utility (such as, but not limited to, acousto-semiconductor,acousto-magnetic, or acousto-optic signal processing utility). Incertain implementations, two or more of the preceding features may becombined.

In one aspect, a micro-electrical-mechanical system (MEMS) guided wavedevice includes a piezoelectric layer, a plurality of electrodesarranged in a slow wave propagation layer disposed below thepiezoelectric layer and configured for transduction of a lateralacoustic wave in the piezoelectric layer; and a guided wave confinementstructure arranged proximate to the slow wave propagation layer andconfigured to confine the lateral acoustic wave in the piezoelectriclayer and the slow wave propagation layer; wherein the piezoelectriclayer includes a first thickness region and a second thickness region,and a thickness of the first thickness region differs from a thicknessof the second thickness region.

In certain embodiments, the piezoelectric layer includes a singlecrystal piezoelectric material. In certain embodiments, the MEMS guidedwave device further includes a bonded interface between thepiezoelectric layer and at least one underlying layer of the MEMS guidedwave device.

In certain embodiments, the plurality of electrodes includes a firstplurality of electrodes arranged against or adjacent to the firstthickness region and configured for transduction of a first lateralacoustic wave having a wavelength λ₁ in the first thickness region, andincludes a second plurality of electrodes arranged against or adjacentto the second thickness region and configured for transduction of asecond lateral acoustic wave having a wavelength λ₂ in the secondthickness region, wherein λ₂ differs from λ₁.

In certain embodiments, the first plurality of electrodes includes afirst interdigital transducer (IDT) including a first two groups ofelectrodes of opposing polarity and including a first spacing betweenadjacent electrodes of opposing polarity of the first two groups ofelectrodes of opposing polarity; the second plurality of electrodesincludes a second interdigital transducer (IDT) including a second twogroups of electrodes of opposing polarity and including a second spacingbetween adjacent electrodes of opposing polarity of the second twogroups of electrodes of opposing polarity; and the second spacingdiffers from the first spacing.

In certain embodiments, one or more electrodes or electrode groupsconfigured for transduction of a lateral acoustic wave in apiezoelectric layer include one or more periodically poled transducers(PPTs).

In certain embodiments, the guided wave confinement structure includes afast wave propagation layer or a Bragg mirror. In certain embodiments,the plurality of electrodes is arranged in contact with thepiezoelectric layer. In certain embodiments, the MEMS guided wave devicefurther includes a substrate underlying the guided wave confinementstructure.

In certain embodiments, the MEMS guided wave device further includes atleast one functional layer at least partially covering the piezoelectriclayer. In certain embodiments, the MEMS guided wave device furtherincludes a first functional layer at least partially covering the firstthickness region, and a second functional layer at least partiallycovering the second thickness region, wherein the second functionallayer differs from the first functional layer in at least one ofmaterial composition, thickness, or material concentration.

In certain embodiments, the MEMS guided wave device further includes atleast one loading material arranged on or proximate to the piezoelectriclayer and configured to locally alter a property of the lateral acousticwave in the piezoelectric layer.

In another aspect, a method of fabricating a MEMS guided wave deviceincludes defining a plurality of electrodes on a piezoelectric layer;depositing a slow wave propagation layer over the plurality ofelectrodes and at least a portion of the piezoelectric layer; providinga guided wave confinement structure on or adjacent to the slow wavepropagation layer, wherein the guided wave confinement structure isconfigured to confine a lateral acoustic wave in the piezoelectric layerand the slow wave propagation layer; and locally thinning thepiezoelectric layer to define a first thickness region and a secondthickness region, wherein a thickness of the first thickness regiondiffers from a thickness of the second thickness region.

In certain embodiments, the method further includes planarizing asurface of the slow wave propagation layer prior to said providing ofthe guided wave confinement structure on or adjacent to the slow wavepropagation layer. In certain embodiments, said local thinning of thepiezoelectric layer includes etching. In certain embodiments, the methodfurther includes depositing at least one of a functional material or aloading material at least partially covering the piezoelectric layerafter said local thinning of the piezoelectric layer.

In another aspect, a method of fabricating a MEMS guided wave deviceincludes defining a plurality of electrodes in a slow wave propagationlayer; bonding or depositing a piezoelectric layer on or over the slowwave propagation layer; and locally thinning the piezoelectric layer todefine a first thickness region and a second thickness region, wherein athickness of the first thickness region differs from a thickness of thesecond thickness region.

In certain embodiments, said defining of the plurality of electrodes inthe slow wave propagation layer includes defining a plurality ofrecesses in the slow wave propagation layer; and depositing electrodematerial in the plurality of recesses. In certain embodiments, themethod further includes planarizing a surface of the slow wavepropagation layer prior to said bonding or depositing of thepiezoelectric layer on or over the slow wave propagation layer.

In another aspect, a micro-electrical-mechanical system (MEMS) guidedwave device includes a piezoelectric layer, a slow wave propagationlayer disposed below the piezoelectric layer, a plurality of electrodesconfigured for transduction of a lateral acoustic wave in thepiezoelectric layer, a guided wave confinement structure arrangedproximate to the slow wave propagation layer and configured to confinethe lateral acoustic wave in the piezoelectric layer and the slow wavepropagation layer; and at least one loading material configured tolocally alter a property of the lateral acoustic wave in thepiezoelectric layer, wherein one of (i) the plurality of electrodes or(ii) the at least one loading material is arranged in the slow wavepropagation layer, and the other one of (i) the plurality of electrodesor (ii) the at least one loading material is arranged over thepiezoelectric layer. In certain embodiments, the plurality of electrodesis arranged in the slow wave propagation layer, and the at least oneloading material is arranged over the piezoelectric layer. In certainembodiments, the plurality of electrodes is arranged over thepiezoelectric layer, and the at least one loading material is arrangedin the slow wave propagation layer.

In certain embodiments, the piezoelectric layer defines a firstthickness region and a second thickness region, wherein a thickness ofthe first thickness region differs from a thickness of the secondthickness region.

In certain embodiments, the at least one loading material includes ametal, a fast wave propagation material, a dielectric material, amagnetic material or a magnetically responsive material, or an epitaxialfilm grown on at least one portion of the piezoelectric layer.

In certain embodiments, the at least one loading material is arranged incontact with an active region of the piezoelectric layer. In certainembodiments, the at least one loading material is arranged in contactwith a non-active region of the piezoelectric layer. In certainembodiments, the MEMS guided wave device further includes at least onefunctional layer arranged over at least a portion of the piezoelectriclayer.

In another aspect, a method of fabricating a MEMS guided wave deviceincludes: arranging one of (i) a plurality of electrodes or (ii) atleast one loading material in a slow wave propagation layer; arrangingthe other one of (i) the plurality of electrodes or (ii) the at leastone loading material over a piezoelectric layer; and providing a guidedwave confinement structure proximate to the slow wave propagation layer;wherein the plurality of electrodes is configured for transduction of alateral acoustic wave in the piezoelectric layer; wherein the guidedwave confinement structure is configured to confine the lateral acousticwave in the piezoelectric layer and the slow wave propagation layer; andwherein the at least one loading material is configured to locally altera property of the lateral acoustic wave transduced in the piezoelectriclayer.

In certain embodiments, the method further includes locally thinning thepiezoelectric layer to define a first thickness region and a secondthickness region, wherein a thickness of the first thickness regiondiffers from a thickness of the second thickness region.

In another aspect, a micro-electrical-mechanical system (MEMS) guidedwave device includes a single crystal piezoelectric layer; a slow wavepropagation layer disposed below the single crystal piezoelectric layer;at least one embedded electrode arranged in the slow wave propagationlayer; at least one top side electrode arranged over the single crystalpiezoelectric layer; and a guided wave confinement structure arrangedproximate to the slow wave propagation layer; wherein either (i) the atleast one embedded electrode or (ii) the at least one top side electrodeincludes a plurality of electrodes configured for transduction of atleast one lateral acoustic wave in the single crystal piezoelectriclayer; wherein the guided wave confinement structure is configured toconfine the at least one lateral acoustic wave in the single crystalpiezoelectric layer and the slow wave propagation layer; and wherein atleast one region of the single crystal piezoelectric layer is sandwichedbetween an embedded electrode of the at least one embedded electrode anda top side electrode of the at least one top side electrode.

In certain embodiments, the MEMS guided wave device further includes aconductive via or trace defined through the single crystal piezoelectriclayer and providing an electrically conductive path between the at leastone top side electrode and the at least one embedded electrode.

In certain embodiments, said at least one region of the single crystalpiezoelectric layer sandwiched between the embedded electrode of the atleast one embedded electrode and the top side electrode of the at leastone top side electrode forms at least one capacitive element.

In certain embodiments, the at least one capacitive element iselectrically coupled in series with the at least one embedded electrode.In certain embodiments, the at least one capacitive element iselectrically coupled in parallel with the at least one embeddedelectrode.

In certain embodiments, the at least one embedded electrode includesfirst and second embedded electrodes that are electrically isolatedrelative to one another, and the at least one top side electrodeincludes first and second top side electrodes that are electricallyisolated relative to one another and arranged to interact with the firstand second embedded electrodes to provide non-contact sensing utility.

In certain embodiments, the plurality of electrodes configured fortransduction of the at least one lateral acoustic wave in the singlecrystal piezoelectric layer includes an interdigital transducer (IDT)including two groups of electrodes of opposing polarity.

In certain embodiments, the single crystal piezoelectric layer includesa first thickness region and a second thickness region, and a thicknessof the first thickness region differs from a thickness of the secondthickness region.

In certain embodiments, the at least one top side electrode isnon-coincident with an active region of the single crystal piezoelectriclayer. In certain embodiments, at least one of a functional material ora loading material is arranged over at least a portion of the singlecrystal piezoelectric layer.

In certain embodiments, the plurality of electrodes configured fortransduction of the at least one lateral acoustic wave in the singlecrystal piezoelectric layer includes a first group of input electrodesconfigured for transduction of a first lateral acoustic wave in thesingle crystal piezoelectric layer and includes a second group of inputelectrodes configured for transduction of a second lateral acoustic wavein the single crystal piezoelectric layer; the embedded electrode andthe top side electrode sandwiching the at least one region of the singlecrystal piezoelectric layer in combination form an output electrode,with the output electrode being positioned laterally between the firstgroup of input electrodes and the second group of input electrodes; andthe first group of input electrodes, the second group of inputelectrodes, and the output electrode are configured to interact with thesingle crystal piezoelectric layer to provide non-linear elasticconvolver utility.

In certain embodiments, the at least one embedded electrode includes thefirst group of input electrodes and the second group of inputelectrodes. In certain embodiments, the at least one top side electrodeincludes the first group of input electrodes and the second group ofinput electrodes. In certain embodiments, the first group of inputelectrodes includes a first interdigital transducer, and the secondgroup of input electrodes includes a second interdigital transducer.

In another aspect, a method of fabricating a MEMS guided wave deviceincludes: providing at least one embedded electrode in a slow wavepropagation layer; arranging a single crystal piezoelectric layer overthe slow wave propagation layer; providing at least one top sideelectrode over the single crystal piezoelectric layer; and providing aguided wave confinement structure proximate to the slow wave propagationlayer; wherein either (i) the at least one embedded electrode or (ii)the at least one top side electrode includes a plurality of electrodesconfigured for transduction of a lateral acoustic wave in the singlecrystal piezoelectric layer; wherein the guided wave confinementstructure is configured to confine the lateral acoustic wave in thesingle crystal piezoelectric layer and the slow wave propagation layer;and wherein one or more regions of the single crystal piezoelectriclayer are sandwiched between (i) one or more embedded electrodes of theat least one embedded electrode and (ii) one or more top side electrodesof the at least one top side electrode.

In another aspect, a micro-electrical-mechanical system (MEMS) guidedwave device includes a single crystal piezoelectric layer; a substrate;a cavity or recess arranged between the substrate and a suspendedportion of the single crystal piezoelectric layer; a first group ofinput electrodes configured for transduction of a first lateral acousticwave in the suspended portion of the single crystal piezoelectric layer;a second group of input electrodes configured for transduction of asecond lateral acoustic wave in the suspended portion of the singlecrystal piezoelectric layer; and an output electrode including a firstplate arranged over the suspended portion of the single crystalpiezoelectric layer and a second plate arranged under the suspendedportion of the single crystal piezoelectric layer; wherein the firstgroup of input electrodes, the second group of input electrodes, and theoutput electrode are configured to interact with the single crystalpiezoelectric layer to provide non-linear elastic convolver utility.

In certain embodiments, the first group of input electrodes and thesecond group of input electrodes are arranged over the suspended portionof the single crystal piezoelectric layer. In certain embodiments, thefirst group of input electrodes and the second group of input electrodesare arranged under the suspended portion of the single crystalpiezoelectric layer

In another aspect, a micro-electrical-mechanical system (MEMS) guidedwave device includes a piezoelectric layer; a plurality of electrodesdisposed below the piezoelectric layer and configured for transductionof at least one lateral acoustic wave in the piezoelectric layer; aguided wave confinement structure configured to confine the at least onelateral acoustic wave in the piezoelectric layer; and at least onesensing material arranged over at least a portion of the piezoelectriclayer; wherein at least one property of the at least one sensingmaterial is configured to change in exposure to an environment proximateto the at least one sensing material.

In certain embodiments, a wave propagation property of the piezoelectriclayer is configured to be altered in response to the change of the atleast one property of the at least one sensing material.

In certain embodiments, the MEMS guided wave device further includes aslow wave propagation layer disposed between the piezoelectric layer andthe guided wave confinement structure, wherein the plurality ofelectrodes is arranged in the slow wave propagation layer.

In certain embodiments, the at least one property of the at least onesensing material is configured to change responsive to: a change ofpresence or concentration of one or more chemical species in theenvironment; a change of presence or concentration of one or morebiological species in the environment; a change of presence or strengthof an electric or magnetic field in the environment; a change oftemperature in the environment; or a change of presence, type, or amountof radiation in the environment.

In certain embodiments, the plurality of electrodes includes aninterdigital transducer (IDT) including two groups of electrodes ofopposing polarity.

In certain embodiments, the at least one sensing material includes afirst sensing material and a second sensing material; wherein the firstsensing material differs from the second sensing material in at leastone of composition, concentration, area, amount, or position.

In certain embodiments, the guided wave confinement structure includes acavity or recess, a portion of the piezoelectric layer is disposed overthe cavity or recess to define a suspended portion of the piezoelectriclayer, and the at least one sensing material is arranged on or over thesuspended portion of the piezoelectric layer.

In another aspect, a micro-electrical-mechanical system (MEMS) guidedwave device includes: a piezoelectric layer; a plurality of electrodesdisposed below the piezoelectric layer and configured for transductionof at least one lateral acoustic wave in the piezoelectric layer; and atleast one underlying layer arranged proximate to the piezoelectric layerand defining a sealed cavity or recess bounded by a suspended portion ofthe piezoelectric layer; wherein a wave propagation property of thepiezoelectric layer is configured to change in response to exposure ofthe piezoelectric layer to a change in pressure of an environmentproximate to the piezoelectric layer.

In certain embodiments, the at least one underlying layer includes aslow wave propagation layer disposed below a portion of thepiezoelectric layer, wherein the plurality of electrodes is arranged inthe slow wave propagation layer.

In certain embodiments, the plurality of electrodes is supported by thesuspended portion of the piezoelectric layer.

In certain embodiments, the underlying layer includes a guided waveconfinement structure configured to confine the at least one lateralacoustic wave in the piezoelectric layer.

In another aspect, a method of fabricating a MEMS guided wave deviceincludes: defining a plurality of electrodes on a first surface of apiezoelectric layer; depositing a slow wave propagation layer over theplurality of electrodes and at least a portion of the first surface ofthe piezoelectric layer; providing a guided wave confinement structureon or adjacent to the slow wave propagation layer; and depositing atleast one sensing material arranged over at least a portion of a secondsurface of the piezoelectric layer; wherein the plurality of electrodesis configured for transduction of at least one lateral acoustic wave inthe piezoelectric layer; wherein the guided wave confinement structureis configured to confine the at least one lateral acoustic wave in thepiezoelectric layer and the slow wave propagation layer; and wherein atleast one property of the at least one sensing material is configured tochange in exposure to an environment proximate to the at least onesensing material.

In one aspect, a micro-electrical-mechanical system (MEMS) mixed domainguided wave device includes: a piezoelectric layer; a plurality ofelectrodes disposed below the piezoelectric layer and configured fortransduction of at least one lateral acoustic wave in the piezoelectriclayer; a guided wave confinement structure configured to confine the atleast one lateral acoustic wave in the piezoelectric layer; and at leastone functional layer arranged on or over at least a portion of thepiezoelectric layer, and configured to interact with the at least onelateral acoustic wave in the piezoelectric layer to provide mixed domainsignal processing utility.

In certain embodiments, the MEMS mixed domain guided wave device furtherincludes a slow wave propagation layer disposed below the piezoelectriclayer, wherein the plurality of electrodes is arranged in the slow wavepropagation layer.

In certain embodiments, the mixed domain signal processing utilityincludes acousto-semiconductor, acousto-magnetic, or acousto-opticsignal processing utility.

In certain embodiments, the at least one functional layer includes oneor more of a conductive material, a semiconducting material, or adielectric material. In certain embodiments, the at least one functionallayer includes one or more of a piezoelectric material, a ferroelectricmaterial, a ferromagnetic material, or a magnetostrictive material. Incertain embodiments, the at least one functional layer includes one ormore of an optically responsive material, a pyroelectric material, or anorganic material.

In certain embodiments, the guided wave confinement structure includes acavity or recess, a portion of the piezoelectric layer is disposed overthe cavity or recess to define a suspended portion of the piezoelectriclayer, and the at least one functional layer is arranged on or over thesuspended portion of the piezoelectric layer.

In certain embodiments, the at least one functional layer includes atleast one semiconducting layer, a first electrical contact arranged overa first portion of the at least one semiconducting layer, and a secondelectrical contact arranged over a second portion of the at least onesemiconducting layer; and the at least one semiconducting layer isconfigured to interact with the piezoelectric layer to provide acousticamplification utility.

In certain embodiments, the at least one functional layer includes afirst semiconducting layer having a first bandgap and a secondsemiconducting layer having a second bandgap that differs from the firstbandgap, at least one electrical contact is in electrical communicationwith at least one of the first semiconducting layer or the secondsemiconducting layer; and the first semiconducting layer and the secondsemiconducting layer form a heterostructure configured to form atwo-dimensional electron gas at an interface between the firstsemiconducting layer and the second semiconducting layer. In certainembodiments, the at least one functional layer includes at least onesemiconducting layer; and a source contact, a gate contact, and a draincontact are operatively arranged with the at least one semiconductinglayer to serve as a transistor.

In certain embodiments, the at least one functional layer includes atleast one semiconducting layer; the plurality of electrodes includes afirst group of input electrodes configured for transduction of a firstlateral acoustic wave in the piezoelectric layer and a second group ofinput electrodes configured for transduction of a second lateralacoustic wave in the piezoelectric layer; an outer conductive layer isarranged over the at least one semiconducting layer, and an innerconductive layer is arranged under a portion of the piezoelectric layer,whereby an output electrode is formed including the outer conductivelayer, the at least one semiconducting layer, the portion of thepiezoelectric layer, and the inner conductive layer; the outputelectrode is positioned laterally between the first group of inputelectrodes and the second group of input electrodes; and the first groupof input electrodes, the second group of input electrodes, and theoutput electrode are configured to interact with the piezoelectric layerto provide acoustoelectric convolver utility.

In certain embodiments, the at least one functional layer includes atleast one semiconducting layer; the plurality of electrodes includes afirst group of input electrodes configured for transduction of a firstlateral acoustic wave in the piezoelectric layer and a second group ofinput electrodes configured for transduction of a second lateralacoustic wave in the piezoelectric layer; output electrodes of opposingpolarity are provided in ohmic contact with the at least onesemiconducting layer; and the first group of input electrodes, thesecond group of input electrodes, and the output electrodes areconfigured to interact with the piezoelectric layer to provide acousticwave convolver with bidirectional amplification utility.

In certain embodiments, the piezoelectric layer comprises a singlecrystal piezoelectric material.

In certain embodiments, at least some electrodes of the plurality ofelectrodes include an interdigital transducer (IDT) including two groupsof electrodes of opposing polarity. In certain embodiments, theplurality of electrodes includes a first interdigital transducer (IDT)including a first two groups of electrodes of opposing polarity, and asecond interdigital transducer (IDT) including a second two groups ofelectrodes of opposing polarity, and the at least a portion of thepiezoelectric layer bearing the at least one functional layer isarranged generally between the first IDT and the second IDT. In certainembodiments, the MEMS mixed domain guided wave device further includes asubstrate underlying the guided wave confinement structure.

In another aspect, a method of fabricating a MEMS mixed domain guidedwave device includes: defining a plurality of electrodes in or on a slowwave propagation layer; bonding or depositing a piezoelectric layer onor over the slow wave propagation layer; and providing at least onefunctional layer arranged on or over at least a portion of thepiezoelectric layer; wherein the plurality of electrodes is configuredfor transduction of at least one lateral acoustic wave in thepiezoelectric layer; and wherein the at least one functional layer isconfigured to interact with the piezoelectric layer to provide mixeddomain signal processing utility.

In certain embodiments, the at least one functional layer includes oneor more of a conductive material, a semiconducting material, adielectric material, a piezoelectric material, a ferroelectric material,a ferromagnetic material, a magnetic material, a magnetostrictivematerial, an optically responsive material, a pyroelectric material, oran organic material.

In certain embodiments, said providing the at least one functional layerincludes depositing, growing, or bonding the at least one functionallayer on or over at least a portion of the piezoelectric layer.

In another aspect, any of the foregoing aspects, and/or various separateaspects and features as described herein, may be combined for additionaladvantage. Any of the various features and elements as disclosed hereinmay be combined with one or more other disclosed features and elementsunless indicated to the contrary herein.

Those skilled in the art will appreciate the scope of the presentinvention and realize additional aspects thereof after reading thefollowing detailed description in association with the accompanyingdrawings.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings illustrate several aspects of the invention,and together with the description serve to explain the principles of theinvention. Features in the drawings are not to scale unless specificallyindicated to the contrary herein.

FIG. 1 is a top plan view of a conventional MEMS guided wave deviceincluding an IDT and corresponding reflector gratings arranged over apiezoelectric layer that overlies a substrate.

FIGS. 2A-2D are cross-sectional views of portions of a MEMS guided wavedevice during various steps of fabrication utilizing a first fabricationmethod, with the resulting MEMS guided wave device (shown in FIG. 2D)including an exposed piezoelectric layer and electrodes embedded in anunderlying layer, useable as a building block for devices according tovarious embodiments of the present disclosure.

FIGS. 3A-3D are cross-sectional views of portions of a MEMS guided wavedevice during various steps of fabrication utilizing a secondfabrication method, with the resulting MEMS guided wave device (shown inFIG. 3D) including an exposed piezoelectric layer and electrodesembedded in an underlying layer, useable as a building block for devicesaccording to various embodiments of the present disclosure.

FIG. 4 is a cross-sectional view of another MEMS guided wave deviceincluding an exposed piezoelectric layer and electrodes embedded in anunderlying layer, useable as a building block for devices according tovarious embodiments of the present disclosure.

FIG. 5 is a cross-sectional view of yet another MEMS guided wave deviceincluding an exposed piezoelectric layer and electrodes embedded in anunderlying layer, useable as a building block for devices according tovarious embodiments of the present disclosure.

FIGS. 6A-6C are cross-sectional views of MEMS guided wave devices, eachincluding an exposed piezoelectric layer and electrodes embedded in anunderlying layer, following selective trimming of each piezoelectriclayer to provide regions of different thicknesses, according to variousembodiments of the present disclosure.

FIG. 7A is a top plan view of a multi-frequency MEMS guided wave deviceincluding an exposed piezoelectric layer, a first group of embeddedelectrodes arranged below a first thickness region of the piezoelectriclayer, and a second group of embedded electrodes arranged below a secondthickness region of the piezoelectric layer, according to one embodimentof the present disclosure.

FIG. 7B is a cross-sectional view of the multi-frequency MEMS guidedwave device of FIG. 7A taken along section line “A”-“A”.

FIG. 8 is a cross-sectional view of a MEMS guided wave device includingan exposed piezoelectric layer and electrodes embedded in an underlyinglayer, with a loading material arranged over the piezoelectric layer andoverlying the group of embedded electrodes, according to one embodimentof the present disclosure.

FIG. 9 is a cross-sectional view of a MEMS guided wave device includingan exposed piezoelectric layer and electrodes embedded in an underlyinglayer, with a loading material arranged over the piezoelectric layerbetween two groups of embedded electrodes, according to one embodimentof the present disclosure.

FIG. 10 is a cross-sectional view of a MEMS guided wave device includingan exposed piezoelectric layer and electrodes embedded in an underlyinglayer, with a first loading material arranged over the piezoelectriclayer over a first group of embedded electrodes, and a second loadingmaterial arranged over the piezoelectric layer over a second group ofembedded electrodes, according to one embodiment of the presentdisclosure.

FIGS. 11A-11D are cross-sectional views of portions of a MEMS guidedwave device during various steps of fabrication, with the resulting MEMSguided wave device (shown in FIG. 11D) including two groups ofelectrodes arranged over a piezoelectric layer, with loading materialsembedded in a slow wave propagation material arranged below thepiezoelectric layer below each group of electrodes, according to oneembodiment of the present disclosure.

FIG. 12A is a top plan view of a multi-frequency MEMS guided wave deviceincluding an exposed piezoelectric layer, a first group of embeddedelectrodes arranged below a first thickness region of the piezoelectriclayer, a first loading material region arranged over a portion of thefirst thickness region, a second group of embedded electrodes arrangedbelow a second thickness region of the piezoelectric layer, and secondloading material regions arranged over portions of the second thicknessregion, according to one embodiment of the present disclosure.

FIG. 12B is a cross-sectional view of the multi-frequency MEMS guidedwave device of FIG. 12A taken along section line “B”-“B”.

FIG. 13A is a top plan view of a MEMS guided wave device including anexposed piezoelectric layer and electrodes embedded in an underlyinglayer, with first and second top side electrodes overlying portions ofthe piezoelectric layer as well as embedded electrodes to providenon-contact capacitive sensing utility, according to one embodiment ofthe present disclosure.

FIG. 13B is a cross-sectional view of the MEMS guided wave device ofFIG. 13A taken along section line “C”-“C”.

FIG. 14A is a top plan view of a MEMS guided wave device including anexposed piezoelectric layer and electrodes embedded in an underlyinglayer, with one exposed top side electrode overlying the piezoelectriclayer as well as an embedded electrode, and with an opening in thepiezoelectric layer to expose an embedded electrode, thereby yielding acapacitor in series with an interdigital transducer, according to oneembodiment of the present disclosure.

FIG. 14B is a cross-sectional view of the MEMS guided wave device ofFIG. 14A taken along section line “D”-“D”.

FIG. 14C is a cross-sectional view of a MEMS guided wave device similarto that of FIGS. 14A and 14B, following addition of contact materialextending through the opening to make an ohmic contact with thepiezoelectric layer.

FIG. 14D is a cross-sectional view of an alternative MEMS guided wavedevice similar to that of FIG. 14C, but wherein first and secondopenings are made through the piezoelectric layer, following addition ofcontact material extending through each opening to make first and secondohmic contacts with the piezoelectric layer without formation of acapacitor element, according to one embodiment of the presentdisclosure.

FIG. 15A is a top plan view of a MEMS guided wave device including anexposed piezoelectric layer and electrodes embedded in an underlyinglayer, with exposed top side electrodes overlying the piezoelectriclayer as well as embedded electrodes, yielding a capacitor in parallelwith an interdigital transducer, according to one embodiment of thepresent disclosure.

FIG. 15B is a cross-sectional view of the MEMS guided wave device ofFIG. 15A taken along section line “E”-“E”.

FIG. 15C is a cross-sectional view of the MEMS guided wave device ofFIG. 15A taken along section line “F”-“F”.

FIG. 16A is a top plan view of a MEMS guided wave device including anexposed piezoelectric layer arranged over electrodes of two resonators(including interdigital transducers and gratings) embedded in anunderlying layer, with sensing material arranged on the piezoelectriclayer over a sensing resonator to provide sensing utility, and with areference resonator being devoid of sensing material arranged over thepiezoelectric layer, according to one embodiment of the presentdisclosure.

FIG. 16B is a cross-sectional view of the MEMS guided wave device ofFIG. 16A taken along section line “G”-“G”.

FIG. 17A is a top plan view of a MEMS guided wave device including anexposed piezoelectric layer arranged over electrodes forming two delaylines (each including two interdigital transducers (IDTs)) embedded inan underlying layer, including a sensor delay line having sensingmaterial overlying the piezoelectric layer and arranged between a firstpair of IDTs, and including a reference delay line being devoid ofsensing material arranged between a second pair of IDTs, according toone embodiment of the present disclosure.

FIG. 17B is a cross-sectional view of the MEMS guided wave device ofFIG. 17A taken along section line “H”-“H”.

FIG. 18 is a top plan view of a MEMS guided wave device including anexposed piezoelectric layer arranged over electrodes forming fourresonators (each including interdigital transducers and gratings)embedded in an underlying layer, with each resonator including adifferent sensing material arranged over the piezoelectric layer betweenpaired interdigital transducers to provide sensing utility, according toone embodiment of the present disclosure.

FIG. 19A is a top plan view of a MEMS guided wave device including anexposed piezoelectric layer with internal electrodes supported by thepiezoelectric layer and forming a resonator (including a pair ofinterdigital transducers arranged between reflector gratings), with aportion of the piezoelectric layer supporting the internal electrodesbeing suspended over a cavity or recess defined in an underlying layerto provide pressure sensing utility, according to one embodiment of thepresent disclosure.

FIG. 19B is a cross-sectional view of the MEMS guided wave device ofFIG. 19A taken along section line “I”-“I”.

FIG. 20A is a top plan view of a MEMS guided wave device including anexposed piezoelectric layer with internal electrodes embedded in anunderlying layer and forming a resonator (including a pair ofinterdigital transducers arranged between reflector gratings), with acentral portion of the piezoelectric layer not in contact with theelectrodes being suspended over a cavity or recess defined in theunderlying layer to provide pressure sensing utility, according to oneembodiment of the present disclosure.

FIG. 20B is a cross-sectional view of the MEMS guided wave device ofFIG. 20A taken along section line “J”-“J”.

FIG. 21A is a top plan view of a MEMS guided wave device including anexposed piezoelectric layer with internal electrodes supported by thepiezoelectric layer and forming a delay line (including a pair oflaterally spaced interdigital transducers), with a portion of thepiezoelectric layer supporting the internal electrodes being suspendedover a cavity or recess defined in an underlying layer, and with afunctional material or sensing material arranged over the suspendedportion of the piezoelectric layer, according to one embodiment of thepresent disclosure.

FIG. 21B is a cross-sectional view of the MEMS guided wave device ofFIG. 21A taken along section line “K”-“K”.

FIG. 22A is a top plan view of a MEMS guided wave device including anexposed piezoelectric layer with internal electrodes embedded in anunderlying layer and forming a delay line (including a pair of laterallyspaced interdigital transducers), with a central portion of thepiezoelectric layer not in contact with the electrodes being suspendedover a cavity or recess defined in the underlying layer, and with afunctional material or sensing material arranged over the suspendedportion of the piezoelectric layer, according to one embodiment of thepresent disclosure.

FIG. 22B is a cross-sectional view of the MEMS guided wave device ofFIG. 22A taken along section line “L”-“L”.

FIG. 23 is a cross-sectional view of a microfluidic device incorporatinga MEMS guided wave device including an exposed piezoelectric layer andelectrodes embedded in an underlying layer, with sensing materialarranged over the piezoelectric layer in an area between twointerdigital transducers to provide sensing utility, according to oneembodiment of the present disclosure.

FIG. 24A is a cross-sectional view of a MEMS guided wave deviceincluding an exposed piezoelectric layer with an internal pair oflaterally spaced interdigital transducers serving as input electrodesand including an output electrode including an internal plate electrodeand an external plate electrode sandwiching a central portion of thepiezoelectric layer, with the MEMS guided wave device suitable forproviding non-linear elastic convolver utility, according to oneembodiment of the present disclosure.

FIG. 24B is a cross-sectional view of a MEMS guided wave deviceincluding a piezoelectric layer with an external pair of laterallyspaced interdigital transducers serving as input electrodes andincluding an output electrode including an internal plate electrode andan external plate electrode sandwiching a central portion of thepiezoelectric layer, with the MEMS guided wave device suitable forproviding non-linear elastic convolver utility, according to oneembodiment of the present disclosure.

FIG. 25A is a cross-sectional view of a MEMS guided wave deviceincluding an exposed piezoelectric layer, with a portion of thepiezoelectric layer being suspended over a cavity or recess in anunderlying layer, including internal electrodes supported by thesuspended portion of piezoelectric layer and forming an internal pair oflaterally spaced interdigital transducers serving as input electrodes,and including an output electrode including an internal plate electrodeand an external plate electrode sandwiching a central portion of thepiezoelectric layer, wherein the MEMS guided wave device is suitable forproviding non-linear elastic convolver utility, according to oneembodiment of the present disclosure.

FIG. 25B is a cross-sectional view of a MEMS guided wave deviceincluding a piezoelectric layer having a portion that is suspended overa cavity or recess in an underlying layer, external electrodes supportedby the suspended portion of piezoelectric layer and forming an externalpair of laterally spaced interdigital transducers serving as inputelectrodes, and an output electrode including an internal plateelectrode and an external plate electrode sandwiching a central portionof the piezoelectric layer, wherein the MEMS guided wave device issuitable for providing non-linear elastic convolver utility, accordingto one embodiment of the present disclosure.

FIG. 26 is a cross-sectional view of a MEMS mixed domain guided wavedevice including an exposed piezoelectric layer, electrodes forming apair of laterally spaced interdigital transducers embedded in anunderlying layer, a semiconducting layer arranged on a central portionof the piezoelectric layer, and first and second ohmic contacts arrangedover portions of the semiconductor layer to provide acousticamplification utility, according to one embodiment of the presentdisclosure.

FIG. 27 is a cross-sectional view of a MEMS mixed domain guided wavedevice including an exposed piezoelectric layer having a portion that issuspended over a cavity or recess in an underlying layer, internalelectrodes supported by the suspended portion of the piezoelectric layerand forming a pair of interdigital transducers, a semiconducting layerarranged on a central portion of the piezoelectric layer, and with firstand second ohmic contacts arranged over portions of the semiconductorlayer to provide acoustic amplification utility, according to oneembodiment of the present disclosure.

FIG. 28 is a cross-sectional view of a MEMS mixed domain guided wavedevice including an exposed piezoelectric layer, internal electrodesforming a pair of laterally spaced interdigital transducers embedded inan underlying layer, and first and second semiconducting layers with atop side electrical contact arranged over a portion of the piezoelectriclayer and configured to form a two-dimensional electron gas at aninterface between the first and second semiconducting layers, accordingto one embodiment of the present disclosure.

FIG. 29 is a cross-sectional view of a MEMS mixed domain guided wavedevice including an exposed piezoelectric layer, internal electrodesforming a pair of laterally spaced interdigital transducers embedded inan underlying layer, and at least one semiconducting layer arranged overa portion of the piezoelectric layer, wherein a source contact, a gatecontact, and a drain contact are operatively arranged with the at leastone semiconducting layer to serve as a transistor, according to oneembodiment of the present disclosure.

FIG. 30 is a cross-sectional view of a MEMS mixed domain guided wavedevice including an exposed piezoelectric layer, internal electrodesforming a pair of laterally spaced interdigital transducers embedded inan underlying layer, an inner conductive layer embedded in theunderlying layer, and multiple layers (including an outer conductivelayer) overlying a portion of the piezoelectric layer to provideacoustoelectric convolver utility, according to one embodiment of thepresent disclosure.

FIG. 31 is a cross-sectional view of a MEMS mixed domain guided wavedevice including an exposed piezoelectric layer, internal electrodesforming a pair of laterally spaced interdigital transducers embedded inan underlying layer, an inner conductive layer embedded in theunderlying layer, and multiple layers (including an outer conductivelayer) overlying a portion of the piezoelectric layer to provideacoustoelectric convolver utility, according to one embodiment of thepresent disclosure.

FIG. 32 is a cross-sectional view of a MEMS mixed domain guided wavedevice including an exposed piezoelectric layer, internal electrodesforming a pair of laterally spaced interdigital transducers embedded inan underlying layer and serving as input electrodes, and asemiconducting layer with top side output electrodes of opposingpolarity overlying a portion of the piezoelectric layer to provideacoustic wave convolver with bidirectional amplification utility,according to one embodiment of the present disclosure.

DETAILED DESCRIPTION

Embodiments set forth below represent the necessary information toenable those skilled in the art to practice the invention and illustratethe best mode of practicing the invention. Upon reading the followingdescription in light of the accompanying drawing figures, those skilledin the art will understand the concepts of the invention and willrecognize applications of these concepts not particularly addressedherein. It should be understood that these concepts and applicationsfall within the scope of the disclosure and the accompanying claims.

It will be understood that, although the terms first, second, etc. maybe used herein to describe various elements, these elements should notbe limited by these terms. These terms are only used to distinguish oneelement from another. For example, a first element could be termed asecond element, and, similarly, a second element could be termed a firstelement, without departing from the scope of the present disclosure. Asused herein, the term “and/or” includes any and all combinations of oneor more of the associated listed items.

Relative terms such as “below” or “above” or “upper” or “lower” or“horizontal” or “vertical” may be used herein to describe a relationshipof one element, layer, or region to another element, layer, or region asillustrated in the Figures. It will be understood that these terms andthose discussed above are intended to encompass different orientationsof the device in addition to the orientation depicted in the Figures.

The terminology used herein is for the purpose of describing particularembodiments only and is not intended to be limiting of the disclosure.As used herein, the singular forms “a,” “an,” and “the” are intended toinclude the plural forms as well, unless the context clearly indicatesotherwise. It will be further understood that the terms “comprises,”“comprising,” “includes,” and/or “including” when used herein specifythe presence of stated features, integers, steps, operations, elements,and/or components, but do not preclude the presence or addition of oneor more other features, integers, steps, operations, elements,components, and/or groups thereof. As used herein, the terms “proximate”and “adjacent” as applied to a specified layer or element refer to astate of being close or near to another layer or element, and encompassthe possible presence of one or more intervening layers or elementswithout necessarily requiring the specified layer or element to bedirectly on or directly in contact with the other layer or elementunless specified to the contrary herein.

Unless otherwise defined, all terms (including technical and scientificterms) used herein have the same meaning as commonly understood by oneof ordinary skill in the art to which this disclosure belongs. It willbe further understood that terms used herein should be interpreted ashaving a meaning that is consistent with their meaning in the context ofthis specification and the relevant art and will not be interpreted inan idealized or overly formal sense unless expressly so defined herein.

Certain aspects of the present disclosure relate to amicro-electrical-mechanical system (MEMS) guided wave device in which aplurality of electrodes are provided (e.g., embedded or otherwiseprovided) below a piezoelectric layer and are configured fortransduction of a lateral acoustic wave in the piezoelectric layer. Incertain embodiments, electrodes are embedded in a slow wave propagationmaterial underlying a piezoelectric layer. In certain embodiments,electrode are supported by a suspended portion of a piezoelectric layer.At least one guided wave confinement structure, such as a fast wavepropagation material, a Bragg mirror, or a cavity or recess, is arrangedproximate to (e.g., below) the slow wave propagation layer and isconfigured to confine the lateral acoustic wave in the piezoelectriclayer and (if provided) the slow wave propagation layer. An optionalsubstrate may be provided below the guided wave confinement structure.

By arranging electrodes below the piezoelectric layer and therebyexposing at least a portion of the piezoelectric layer, a “buildingblock” structure is formed in which a surface of the piezoelectric layeris available to permit one or more additions or modifications to be madethereto, thereby enabling production of devices with enhanced utility,as detailed herein. As one example, one or more regions of apiezoelectric layer may be selectively thinned (or “trimmed”). Asanother example, one or more loading materials may be arranged incontact with one or more portions of a piezoelectric layer to locallyalter a property of a lateral acoustic wave transduced in thepiezoelectric layer. As another example, one or more regions of apiezoelectric layer may be sandwiched between one or more embeddedelectrodes and one or more top side electrodes to yield one or morecapacitive elements. As another example, at least one sensing materialmay be arranged over at least a portion of the piezoelectric layer,wherein at least one property of at least one sensing material isconfigured to change in exposure to an environment proximate to the atleast one sensing material, and at least one wave propagation propertyof the piezoelectric layer may be altered in response to such change. Asanother example, at least one functional layer may be arranged on orover at least a portion of the piezoelectric layer, and configured tointeract with the piezoelectric layer to provide mixed domain signalprocessing utility (such as, but not limited to, acousto-semiconductor,acousto-magnetic, or acousto-optic signal processing utility).

While certain embodiments utilize internal electrodes that are eitherembedded in a layer underlying a piezoelectric layer or are supported bya suspended portion of a piezoelectric layer, in other embodiments, atleast some electrodes may be externally arranged over a piezoelectriclayer. In certain embodiments, first and second electrode groups (e.g.,interdigital transducers) may be externally arranged over apiezoelectric layer, and at least one additional electrode may beembedded below or otherwise provided below the piezoelectric layer.

In certain embodiments, MEMS guided wave devices described herein mayhave dominant lateral vibrations. Such devices may desirably use singlecrystal piezoelectric layer materials, such as lithium tantalate orlithium niobate. Such devices may also provide vibrating structures withprecise sizes and shapes, which may provide high accuracy, and enablefabrication of multiple resonators having different resonant frequencieson a single substrate. Although lithium niobate and lithium tantalateare particularly preferred piezoelectric materials, in certainembodiments any suitable piezoelectric materials may be used, such asquartz, a piezoceramic, or a deposited piezoelectric material (such asaluminum nitride or zinc oxide). When provided, substrates of anysuitable materials may be used, such as silicon, glass, ceramic, etc. Incertain embodiments, a substrate may additionally or alternativelycomprise a piezoelectric material, which may be of the same or differentcomposition in comparison to the piezoelectric layer against whichelectrodes are provided for transduction of one or more acoustic waves.

Vibrating structures of preferred MEMS guided wave devices describedherein are formed of single crystal piezoelectric material and usemechanically efficient MEMS construction. Such vibrating structures maybe high-Q, low loss, stable, have a high electromechanical couplingcoefficient, have high repeatability, and have a low motional impedance.In certain embodiments, a nonstandard (e.g., offcut) crystallineorientation of the single crystal piezoelectric material may be used toprovide specific vibrational characteristics, such as a low temperaturecoefficient of frequency, a high electromechanical coupling coefficient,or both. Since it is extremely difficult to grow single crystalpiezoelectric material (e.g., via epitaxy) over non-lattice-matchedmaterials, in certain embodiments, single crystal piezoelectricmaterials are prefabricated (e.g., by growth of a boule followed byformation of thin wafers), surface finished (e.g., via chemicalmechanical planarization (CMP) and polishing to provide near-atomicflatness), and bonded to one or more underlying layers. Any suitablewafer bonding technique known in the art may be used, such as may relyon van der Waals bonds, hydrogen bonds, covalent bond, and/or mechanicalinterlocking. In certain embodiments, direct bonding may be used. Incertain embodiments, bonding may include one or more surface activationsteps (e.g., plasma treatment, chemical treatment, and/or othertreatment methods) followed by application of heat and/or pressure,optionally followed by one or more annealing steps. Such bonding resultsin formation of a bonded interface between the piezoelectric layer andat least one underlying layer. In certain embodiments, the bondedinterface may include at least one intervening layer arranged on atleast a portion of (or the entirety of) a surface of the substrate.

In certain embodiments, a composite including a single crystalpiezoelectric layer, a slow wave propagation layer having embeddedelectrodes, and at least one guided wave confinement structure(optionally in combination with one or more additional layers providingslow wave propagation and/or temperature compensation utility asdisclosed herein) is solidly mounted to a carrier substrate. In otherembodiments, at least a portion of a composite may be suspended above acarrier substrate with a gap arranged therebetween. In certainembodiments, devices described herein may be used for propagation ofquasi-shear horizontal waves, quasi-longitudinal waves, and/orthickness-extensional (FBAR-type) waves. In certain embodiments, atleast one portion of a piezoelectric layer is suspended over one or moreunderlying layers, such as over at least one cavity or recess defined inone or more underlying layers.

The terms “fast wave propagation material” or “fast wave propagationlayer” refers to a material or layer in which an acoustic wave ofinterest travels more quickly than in a proximate piezoelectric layer inwhich the acoustic wave is transduced. Similarly, the terms “slow wavepropagation material” or “slow wave propagation layer” refers to amaterial or layer in which an acoustic wave of interest travels moreslowly than in a proximate piezoelectric layer in which the acousticwave is transduced. Examples of fast wave propagation materials that maybe used according to certain embodiments include (but are not limitedto) diamond, sapphire, aluminum nitride, silicon carbide, boron nitride,and silicon. An example of a slow wave propagation material that may beused according to certain embodiments includes (but is not limited to)silicon dioxide (SiO₂). Silicon dioxide may also be used as asacrificial material in certain embodiments. In certain embodiments,fast wave propagation material may be provided below a slow wavepropagation layer in contact with a piezoelectric layer to confine alateral acoustic wave in the piezoelectric layer and the slow wavepropagation layer, thereby serving as a guided wave confinementstructure. Such confinement may significantly reduce or avoiddissipation of acoustic radiation in an optionally provided substrate orother carrier structure.

Certain embodiments disclosed herein utilize acoustic Bragg mirrors(also known as Bragg reflectors) to serve as a guided wave confinementstructure. A Bragg mirror includes at least one group of at least onelow impedance layer (e.g., silicon dioxide) and at least one highimpedance layer (e.g., tungsten or hafnium dioxide), wherein the atleast one low impedance layer is sequentially arranged with the at leastone high impedance layer in the at least one group. The number of groupsof alternating impedance layers used in a Bragg mirror depends on thetotal reflection coefficient required. In certain embodiments, a Braggmirror may be provided below a slow wave propagation layer in contactwith a piezoelectric layer to confine a lateral acoustic wave in thepiezoelectric layer and the slow wave propagation layer, thereby servingas a guided wave confinement structure.

Certain embodiments disclosed herein utilize a cavity or recess definedin at least one layer underlying a piezoelectric layer to serve as aguided wave confinement structure. In certain embodiments, a compositeincluding a piezoelectric layer and at least one underlying layer (e.g.,a slow wave propagation layer and/or a fast wave propagation layer orBragg mirror) is suspended over a cavity, recess, or void.

In certain embodiments, MEMS guided wave devices may include one or oneor more delay lines, wherein each delay line includes first and secondelectrode groups (e.g., first and second IDTs) that are laterally spacedrelative to one another. In certain embodiments, a first electrode groupis arranged to generate at least one lateral acoustic wave in apiezoelectric layer, and a second electrode group is arranged to receivethe lateral acoustic wave following transmission through thepiezoelectric layer.

In certain embodiments, MEMS guided wave devices may include one or moreresonators, wherein each resonator includes first and second electrodegroups (e.g., first and second IDTs) that are laterally spaced relativeto one another, and wherein the electrode groups are further arrangedbetween reflectors (e.g., reflector gratings).

In certain embodiments, multiple resonators or delay lines of differentwavelengths may be provided on a single wafer or substrate. Suitableelectrodes may be defined on and/or in the piezoelectric layer fortransduction of a first lateral acoustic wave in a first region of thepiezoelectric material, and for transduction of a second lateralacoustic wave in a second region of the piezoelectric material. Incertain embodiments, at least two different resonators or delay lines ona single wafer or substrate are configured to produce wavelengths atleast one octave apart. In certain embodiments, at least one firstresonator or delay line is configured to operate at or around 900 MHz,and at least one second resonator or delay line is configured to operateat or around 1800 MHz or 2.4 GHz.

In certain embodiments, vertical holes may be defined in a piezoelectriclayer (preferably spaced apart from electrodes thereon or thereunder) toenable passage of one or more liquids suitable to promote removal ofsacrificial material arranged below the piezoelectric material.Optionally, such vertical holes may be covered or plugged (e.g., withepoxy or another suitable material) after removal of sacrificialmaterial is complete to promote formation of a sealed cavity or recess.In certain embodiments, lateral access may be provided to sacrificialmaterial arranged within or below a piezoelectric layer, therebyobviating the need for vertical holes to enable removal of sacrificialmaterial.

Guided wave devices as disclosed herein may incorporate variouscombinations of electrode configurations as illustrated in the drawingsand described herein. Exemplary configurations disclosed herein include,but are not limited to, interdigital transducers (IDTs) or periodicallypoled transducers (PPTs) alone, and IDTs or PPTs in combination withelectrodes of other types, such as plate electrodes, or continuous layer(e.g., floating) electrodes. An IDT includes electrodes with a firstconducting section and a second conducting section that areinter-digitally dispersed in or on a surface or layer. IDTs are wellknown in the art, and may be defined by single-step or multi-stepphotolithographic patterning.

Although various figures herein include one or more input electrodes,output electrodes, IDTs, reflector gratings, resonators, delay lines,filters, sensors, capacitors, and/or mixed domain signal processingelements in discrete fashion, it is to be appreciated that any suitablecombinations of input electrodes, output electrodes, IDTs, reflectorgratings, resonators, delay lines, filters, sensors, capacitors, and/ormixed domain signal processing elements in series and/or in parallel maybe provided on a single wafer or substrate. In certain embodiments,multiple delay lines, resonators, and/or filters arranged fortransduction of acoustic waves of different wavelengths may be providedin a single wafer or substrate, optionally in conjunction with one ormore other elements or features disclosed herein.

As noted previously, a “building block” structure according to certainembodiments may include electrodes arranged below a piezoelectric layer,whereby a surface of the piezoelectric layer is available to permit oneor more additions or modifications to be made thereto.

FIGS. 2A-2D are cross-sectional views of portions of a MEMS guided wavedevice during various steps of fabrication utilizing a first fabricationmethod, with the device of FIG. 2D being useable as a “building block”structure. FIG. 2A illustrates formation of a first group of alternatingelectrodes 38, 40 (e.g., forming a first IDT) and a second group ofalternating electrodes 38A, 40A (e.g., forming a second IDT) over asurface of a thick piezoelectric layer 30. Such electrodes 38, 40, 38A,40A may be formed by conventional methods such as metal depositionfollowed by photolithographic patterning and etching. In certainembodiments, the piezoelectric layer 30 comprises a single crystalpiezoelectric material. FIG. 2B illustrates the structure of FIG. 2Afollowing deposition of a slow wave propagation layer 26 over theelectrodes 38, 40, 38A, 40A and one surface of the thick piezoelectriclayer 30. Thereafter, the structure of FIG. 2B may be inverted, and aguided wave confinement structure 28 may be provided in contact with theslow wave propagation layer 26, as shown in FIG. 2C. In certainembodiments, a surface of the slow wave propagation layer 26 may beplanarized (e.g., via chemical mechanical planarization or anothersuitable process) before the guided wave confinement structure 28 isprovided thereon. Any suitable process may be used for providing theguided wave confinement structure 28, such as deposition, directbonding, adhesion, or other means. In certain embodiments, one or moresurface activation steps may be applied to the slow wave propagationlayer 26 and/or the guided wave confinement structure 28 prior to directbonding or adhesion. One or more bonding layers may optionally bearranged between the respective layers 26, 28 to be bonded. Thereafter,the piezoelectric layer 30 may be thinned (e.g., via one or more ofetching, lapping, grinding, planarization, polishing, etc.) to a desiredthickness to yield an exposed upper surface 30′, as shown in FIG. 2D.The exposed upper surface 30′ is subject to one or more addition and/ormodification steps as described herein. For example, in certainembodiments, the piezoelectric layer 30 may be locally thinned to definea first thickness region and a second thickness region, wherein athickness of the first thickness region differs from a thickness of thesecond thickness region.

FIGS. 3A-3D are cross-sectional views of portions of a MEMS guided wavedevice during various steps of fabrication utilizing a secondfabrication method, with the device of FIG. 3D being useable as such a“building block” structure. FIG. 3A illustrates a slow wave propagationlayer 26 arranged (e.g., via deposition or bonding) on a guided waveconfinement structure 28 (e.g., a fast wave propagation layer or a Braggmirror). FIG. 3B illustrates the structure of FIG. 3A followingformation of a photoresist layer 34 over the slow wave propagation layer26, photolithographic patterning of the photoresist layer 34 to definewindows 33, and etching to form recesses 32 in the slow wave propagationlayer 26. FIG. 3C illustrates the structure of FIG. 3B following removalof the photoresist layer 34 and following deposition in the recesses 32of alternating electrodes 38, 40 forming a first IDT and alternatingelectrodes 38A, 40A forming a second IDT. The electrodes 38, 40, 38A,40A are recessed in the slow wave propagation layer 26. Thereafter, asshown in FIG. 3D, a piezoelectric layer 30 is provided over the slowwave propagation layer 26 and the electrodes 38, 40, 38A, 40A. Incertain embodiments, the piezoelectric layer 30 may be deposited orgrown over the slow wave propagation layer 26 and the electrodes 38, 40,38A, 40A. In other embodiments, the piezoelectric layer 30 (preferablyembodying a single crystal piezoelectric material) may be prefabricated.Adjacent surfaces of the piezoelectric layer 30 and the slow wavepropagation layer 26 may be planarized and polished, and then attachedto one another via a conventional direct bonding (e.g., wafer bonding)process or other process. One or more bonding promoting layers mayoptionally be arranged between the respective layers 26, 30 to bebonded. Following bonding, the piezoelectric layer 30 includes anexposed upper surface 30′ that may be planarized and polished, and thatis subject to one or more addition and/or modification steps asdescribed herein.

FIG. 4 is a cross-sectional view of another MEMS guided wave deviceincluding a piezoelectric layer 30 having an exposed upper surface 30′,and electrodes 38, 40, 38A, 40A embedded in an underlying slow wavepropagation layer 26. A guided wave confinement structure 28 (e.g., afast wave propagation layer) is provided below the slow wave propagationlayer 26, and a substrate 36 underlies the guided wave confinementstructure 28. The structure shown in FIG. 4 is useable as a buildingblock for devices according to various embodiments of the presentdisclosure.

FIG. 5 is a cross-sectional view of a MEMS guided wave device similar tothat shown in FIG. 4, but utilizing a guided wave confinement structure28 in the form of a Bragg mirror instead of a fast wave propagationlayer. The Bragg mirror includes alternating low and high impedancelayers 28A-28C. A piezoelectric layer 30 has an exposed upper surface30′, and electrodes 38, 40, 38A, 40A are embedded in an underlying slowwave propagation layer 26. The guided wave confinement structure 28 isprovided below the slow wave propagation layer 26, and a substrate 36underlies the guided wave confinement structure 28. The structure shownin FIG. 5 is useable as a building block for devices according tovarious embodiments of the present disclosure.

In certain embodiments, selective areas of piezoelectric layers may betrimmed (via one or more appropriate techniques such as, but not limitedto, ion milling and/or etching) to provide different thickness regions,thereby permitting adjustment of frequency and/or coupling strength, andenabling formation of multi-frequency devices (e.g., when electrodes ofdifferent periodicities are provided over a single wafer or substrate).In certain embodiments, two, three, four or more different thicknessregions of a single piezoelectric layer may be provided. In certainembodiments, multiple regions of a first thickness of a piezoelectriclayer may be continuous or discontinuous.

In certain embodiments, a micro-electrical-mechanical system (MEMS)guided wave device includes a piezoelectric layer, a plurality ofelectrodes arranged in a slow wave propagation layer disposed below thepiezoelectric layer and configured for transduction of a lateralacoustic wave in the piezoelectric layer; and a guided wave confinementstructure arranged proximate to the slow wave propagation layer andconfigured to confine the lateral acoustic wave in the piezoelectriclayer and the slow wave propagation layer; wherein the piezoelectriclayer includes a first thickness region and a second thickness region,and a thickness of the first thickness region differs from a thicknessof the second thickness region.

FIG. 6A illustrates a MEMS guided wave device including a piezoelectriclayer 30 overlying electrodes 38, 40, 38A, 40A embedded in an underlyingslow wave propagation layer 26 that overlies a guided wave confinementstructure 28. The piezoelectric layer 30 includes an exposed uppersurface 30′. Following local thinning (or trimming) of the piezoelectriclayer 30 (e.g., via etching, ion milling, and/or another suitabletechnique), the piezoelectric layer 30 includes a first thickness region42 and a second thickness region 44, wherein a thickness of the firstthickness region 42 differs from (i.e., is smaller than) a thickness ofthe second thickness region 44. As shown in FIG. 6A, the first thicknessregion 42 overlies a first group of embedded electrodes 38A, 40A as wellas a central region of the piezoelectric layer 30, and the secondthickness region 44 overlies a second group of embedded electrodes 38,40.

FIG. 6B illustrates another MEMS guided wave device including apiezoelectric layer 30 having an exposed upper surface 30′, wherein thepiezoelectric layer 30 overlies electrodes 38, 40, 38A, 40A embedded inan underlying slow wave propagation layer 26 that overlies a guided waveconfinement structure 28. Following local thinning (or trimming) of thepiezoelectric layer 30 (e.g., via etching, ion milling, and/or anothersuitable technique), the piezoelectric layer 30 includes a firstthickness region 42 and a second thickness region 44, wherein athickness of the first thickness region 42 differs from (i.e., issmaller than) a thickness of the second thickness region 44. As shown inFIG. 6B, the second thickness region 44 overlies a first group ofembedded electrodes 38A, 40A and a second group of embedded electrodes38, 40, and the first thickness region 42 overlies a central region ofthe piezoelectric layer 30 between the respective groups of electrodes38, 40, 38A, 40A.

FIG. 6C illustrates another MEMS guided wave device including apiezoelectric layer 30 having an exposed upper surface 30′, wherein thepiezoelectric layer 30 overlies electrodes 38, 40, 38A, 40A embedded inan underlying slow wave propagation layer 26 that overlies a guided waveconfinement structure 28. Following local thinning (or trimming) of thepiezoelectric layer 30 (e.g., via etching, ion milling, and/or anothersuitable technique), the piezoelectric layer 30 includes a firstthickness region 42 and a second thickness region 44, wherein athickness of the first thickness region 42 differs from (i.e., issmaller than) a thickness of the second thickness region 44. As shown inFIG. 6C, the second thickness region 44 overlies a central region of thepiezoelectric layer 30 between the respective groups of electrodes 38,40, 38A, 40A, and the first thickness region 42 overlies a first groupof embedded electrodes 38A, 40A and well as a second group of embeddedelectrodes 38, 40.

In certain embodiments, selective thinning (or trimming) of apiezoelectric layer may be performed over one or more bus bars arrangedbelow the piezoelectric layer (e.g., embedded in an underlying slow wavepropagation layer or supported by a suspended portion of thepiezoelectric layer).

FIG. 7A provides a top plan view, and FIG. 7B provides a cross-sectionalview, of a multi-frequency MEMS guided wave device 60 including apiezoelectric layer 30 having an exposed upper surface 30′ and first andsecond thickness regions 42, 44 separated by a step 46. Thepiezoelectric layer 30 (which may preferably be a single crystalpiezoelectric material) overlies a slow wave propagation layer 26 havingelectrodes embedded therein, with the slow wave propagation layeroverlying a guided wave confinement structure 28 (e.g., a fast wavepropagation layer or a Bragg mirror). A first group of embeddedelectrodes forms a resonator including a first IDT 58-1 and two firstreflectors 50-1 (each including bus bars 52-1 and fingers 54-1) embeddedin the slow wave propagation layer 26 below the first thickness region42. A second group of embedded electrodes forms a resonator including asecond IDT 58-2 and two second reflectors 50-2 (each including bus bars52-2 and fingers 54-2) embedded in the slow wave propagation layer 26below the second thickness region 44. The operating frequency of eachresonator of the MEMS guided wave device 60 is a function of the pitchrepresenting the spacing between fingers 54-1, 54-2 of each respectiveIDT 58-1, 58-2. A first pitch (P1) represents the spacing betweenfingers 54-1 of the first IDT 58-1. A second pitch (P2) represents thespacing between fingers 54-2 of the second IDT 58-2. The wavelength λ ofan acoustic wave transduced by an IDT equals two times the pitch orseparation distance between adjacent electrodes (fingers) of oppositepolarity, and the wavelength λ also equals the separation distancebetween closest electrodes (fingers) of the same polarity. Lateral modedevices have preferred thickness ranges for a piezoelectric layer topromote efficient excitation of lateral waves, such that trimming of oneor more portions of a piezoelectric layer may be used to alter deviceperformance. Since the first and second IDTs 58-1, 58-2 are intended fortransduction of lateral acoustic waves having wavelengths that differfrom one another, the IDTs 58-1, 58-2 are provided under first andsecond thickness regions 42, 44 of the piezoelectric layer 30,respectively.

In certain embodiments, performance of a MEMS guided wave deviceincluding a piezoelectric layer may be altered by providing one or moreloading materials arranged in contact with one or more portions of apiezoelectric layer to locally alter a property of a lateral acousticwave transduced in the piezoelectric layer. In certain embodiments, oneor more loading materials are arranged in contact with an active regionof the piezoelectric layer (e.g., over or between electrodes orelectrode groups). In other embodiments, one or more loading materialsare arranged in contact with a non-active region of the piezoelectriclayer, such as in one or more regions outside intended propagation oflateral acoustic waves. In certain embodiments, loading may be employedwithout trimming, or in combination with trimming. In certain instances,large wavelength adjustments may be accomplished via trimming, andsmaller wavelength adjustments may be accomplished via loading. Examplesof properties that may be affected by addition of one or more loadingmaterials include frequency, coupling coefficient, temperaturecompensation characteristics, velocity, capacitance, or propagative wavemode. In certain embodiments, trimming and loading may be used incombination in a single MEMS guided wave device. Examples of loadingmaterials that may be used include, but are not limited to: metals(e.g., not serving as electrodes), fast wave propagation materials,dielectric materials, magnetic materials, magnetically responsivematerials, and epitaxial films. Loading materials may be applied by anysuitable methods, such as direct bonding, deposition, coating,sputtering, adhering, etc. In certain embodiments, multiple loadingmaterials and/or multiple loading material regions may be provided overa piezoelectric layer. In certain embodiments, loading material regionsmay differ in one or more of the following respects: materialcomposition, material density, thickness, volume, top area, patterning,and/or placement.

The terms “loading material” or “loading layer” as used herein refer toa material or layer configured to change at least one property of anacoustic wave in a manner that is not subject to substantial change withtime, and that preferably undergo only minimal change with exposure toan environment. In contrast, the terms “functional material” or“functional layer” as used herein refer to a material or layer capableof dynamically changing at least one property of an acoustic wave. Theterms “sensing material” or “sensing layer” as used herein refer to amaterial or layer having at least one property that is configured tochange in exposure to a specified environment or environmentalcondition.

In certain embodiments, a MEMS guided wave device includes apiezoelectric layer; a slow wave propagation layer disposed below thepiezoelectric layer; a plurality of electrodes configured fortransduction of a lateral acoustic wave in the piezoelectric layer; aguided wave confinement structure arranged proximate to the slow wavepropagation layer and configured to confine the lateral acoustic wave inthe piezoelectric layer and the slow wave propagation layer; and atleast one loading material configured to locally alter a property of thelateral acoustic wave in the piezoelectric layer. In certainembodiments, the plurality of electrodes is arranged in the slow wavepropagation layer, and at least one loading material is arranged overthe piezoelectric layer. Such a device may utilize a “building block”having an exposed piezoelectric layer such as disclosed in FIG. 2D, 3D,4, or 5. In alternative embodiments, at least one loading material isarranged in the slow wave propagation layer, and the plurality ofelectrodes is arranged over the piezoelectric layer (such that at leasta portion of the piezoelectric layer is covered with electrodes).

FIGS. 8-10 depict embodiments in which at least one loading materialregion is provided over an exposed surface of a piezoelectric layer thatis arranged over electrodes embedded in an underlying layer.

FIG. 8 illustrates a MEMS guided wave device including a piezoelectriclayer 30 having an exposed upper surface 30′, with the piezoelectriclayer 30 overlying electrodes 38, 40, 38A, 40A embedded in a slow wavepropagation layer 26. A guided wave confinement structure 28 (e.g., afast wave propagation layer) is provided below the slow wave propagationlayer 26. A loading material 62 is arranged over and in contact with aportion of the piezoelectric layer 30, generally overlying theelectrodes 38A, 40A.

FIG. 9 illustrates a MEMS guided wave device including a piezoelectriclayer 30 having an exposed upper surface 30′, with the piezoelectriclayer 30 overlying electrodes 38, 40, 38A, 40A embedded in a slow wavepropagation layer 26 that overlies a guided wave confinement structure28 (e.g., a fast wave propagation layer). A loading material 62 isarranged over and in contact with a central portion of the piezoelectriclayer 30, arranged generally between groups of electrodes 38, 40, 38A,40A.

Although FIGS. 8 and 9 illustrate a loading material 62 arranged over anelectrode group or between electrode groups, it is to be appreciatedthat in certain embodiments, one or more loading materials may bearranged in a substantially continuous manner over and between electrodegroups, such as to cover substantially an entire resonator or delayline. In certain embodiments, one or more loading materials may besymmetrically arranged over one or more portions of a resonator or delayline. In certain embodiments, one or more loading materials may bearranged solely over bus bar portions or finger portions of a resonatoror delay line.

In certain embodiments, multiple loading material regions may beprovided. FIG. 10 illustrates a MEMS guided wave device including apiezoelectric layer 30 having an exposed upper surface 30′, with thepiezoelectric layer 30 overlying electrodes 38, 40, 38A, 40A embedded ina slow wave propagation layer 26 that overlies a guided wave confinementstructure 28 (e.g., a fast wave propagation layer). A first loadingmaterial region 62A is arranged over and in contact with a first portionof the piezoelectric layer 30 overlying a first group of electrodes 38,40, and a second loading material region 62B is arranged over and incontact with a second portion of the piezoelectric layer 30 overlying asecond group of electrodes 38A, 40A. In this matter, a property of alateral acoustic wave transduced in the piezoelectric layer 30 may bealtered at different locations. As shown in FIG. 10, thickness ofdifferent loading material regions 62A, 62B may differ (e.g., with thefirst loading material region 62A being thicker than the second loadingmaterial region 62B). In certain embodiments, loading material regions62A, 62B may differ with respect to one or more suitablecharacteristics, such as thickness, concentration, area, patterning,placement, or the like.

FIGS. 8-10 illustrate embodiments in which at least one loading materialregion is provided over an exposed surface of a piezoelectric layer thatis arranged over electrodes embedded in an underlying layer.

In certain embodiments, at least one loading material may be arranged(e.g., embedded) in a slow wave propagation layer, and a plurality ofelectrodes may be arranged over a piezoelectric layer of a MEMS guidedwave device. FIGS. 11A-11D are cross-sectional views of portions of an“internally loaded” MEMS guided wave device during various steps offabrication.

FIG. 11A illustrates a slow wave propagation layer 26 arranged (e.g.,via deposition or bonding) on a guided wave confinement structure 28(e.g., a fast wave propagation layer or a Bragg mirror), followingapplication of a photoresist layer 34 over the slow wave propagationlayer 26, photolithographic patterning of the photoresist layer 34 todefine windows 33, 33A and etching to form recesses 32, 32A in the slowwave propagation layer 26. FIG. 11B illustrates the structure of FIG.11A following addition of loading materials 62, 62A to the recesses 32,32A, and following removal of the photoresist layer 34. Thereafter, asshown in FIG. 11C, a piezoelectric layer 30 is provided over the slowwave propagation layer 26 and the loading materials 62, 62A. In certainembodiments, the piezoelectric layer 30 may be deposited or grown overthe slow wave propagation layer 26 and the loading materials 62, 62A. Inother embodiments, the piezoelectric layer 30 (preferably embodying asingle crystal piezoelectric material) may be prefabricated. Adjacentsurfaces of the piezoelectric layer 30 and a composite including theslow wave propagation layer 26 and the loading materials 62, 62A may beplanarized and polished, and then attached to one another via aconventional direct bonding (e.g., wafer bonding) process or otherprocess. One or more bonding promoting layers may optionally be arrangedbetween the layers to be bonded. Following bonding, the piezoelectriclayer 30 includes an exposed upper surface 30′ on which electrodes 38,40, 38A, 40A are provided, as shown in FIG. 11D. A first group ofelectrodes 38, 40 is arranged in contact with the piezoelectric layer 30generally above the first embedded loading material 62, and a secondgroup of electrodes 38A, 40A is arranged in contact with thepiezoelectric layer 30 generally above the second embedded loadingmaterial 62A, with each loading material 62, 62A being in contact with alower surface 30″ of the piezoelectric layer 30.

FIG. 12A is a top plan view, and FIG. 12B is a cross-sectional view, ofa multi-frequency MEMS guided wave device 60′ including a piezoelectriclayer 30 having an exposed upper surface 30′, first and second thicknessregions 42, 44 separated by a step 46, and being overlaid with multipleloading materials 62-1, 62-2, 62A-2. The piezoelectric layer 30 (whichmay preferably be a single crystal piezoelectric material) overlies aslow wave propagation layer 26 having electrodes embedded therein, withthe slow wave propagation layer 26 overlying a guided wave confinementstructure 28 (e.g., a fast wave propagation layer or a Bragg mirror). Afirst group of embedded electrodes forms a resonator including a firstIDT 58-1 and two first reflectors 50-1 (each including bus bars 52-1 andfingers 54-1) embedded in the slow wave propagation layer 26 below thefirst thickness region 42 and in contact with the piezoelectric layer30. A second group of embedded electrodes forms a resonator including asecond IDT 58-2 and two second reflectors 50-2 (each including bus bars52-2 and fingers 54-2) embedded in the slow wave propagation layer 26below the second thickness region 44 and in contact with thepiezoelectric layer 30. A first pitch (P1) represents the spacingbetween fingers 54-1 of the first IDT 58-1, and a second pitch (P2)represents the spacing between fingers 54-2 of the second IDT 58-2.

Relative to the first thickness region 42 of the piezoelectric layer 30,a first loading material 62-1 is arranged over fingers 54-1 of the firstIDT 58-1 and fingers 54-1 of the reflectors 50-1, without overlappingthe bus bars 52-1 thereof. Although FIG. 12A shows the first loadingmaterial 62-1 as spanning the fingers 54-1 of the first IDT 58-1 andfingers 54-1 of the reflectors 50-1 in a substantially continuousfashion, it is to be appreciated that in certain embodiments, the firstloading material 62-1 may be altered in size or shape, or may beseparated into multiple discrete (non-connected) loading materials, suchas one loading material covering fingers 54-1 of the first IDT 58-1, andseparate loading materials covering fingers 54-1 of the reflectors 50-1.Relative to the second thickness region 44 of the piezoelectric layer30, additional loading materials 62-2, 62A-2 are arranged over bus bars52-2 of the second IDT 58-2 and bus bars 52-2 of the reflectors 50-2,without overlapping the fingers 54-2 thereof. Although FIG. 12A showsthe additional loading materials 62-2, 62A-2 as spanning the bus bars52-2 of the second IDT 58-2 and bus bars 52-2 of the reflectors 50-2 ina substantially continuous fashion, it is to be appreciated that incertain embodiments, the additional loading materials 62-2, 62A-2 may bealtered in size or shape, or may be separated into multiple discrete(non-connected) loading materials, wherein each may include one loadingmaterial covering bus bars 52-2 of the second IDT 58-2, and separateloading materials covering bus bars 52-2 of the reflectors 50-2. A firstacoustic wave having a first wavelength may be transduced by the firstIDT 58-1 in the first thickness region 42, and presence of the firstloading material 62-1 may alter at least one characteristic of suchwave. A second acoustic wave having a second wavelength may betransduced by the second IDT 58-2 in the second thickness region 44, andpresence of the loading materials 62-2, 62A-2 may alter at least onecharacteristic of such wave. As will be appreciated by one skilled inthe art, any suitable number, placement, patterning, and combination ofloading materials may be used.

Thus, consistent with the preceding figures directed to selectivelyloaded piezoelectric layers, a MEMS guided wave device according tocertain embodiments includes: a piezoelectric layer; a slow wavepropagation layer disposed below the piezoelectric layer; a plurality ofelectrodes configured for transduction of a lateral acoustic wave in thepiezoelectric layer; a guided wave confinement structure arrangedproximate to the slow wave propagation layer and configured to confinethe lateral acoustic wave in the piezoelectric layer and the slow wavepropagation layer; and at least one loading material configured tolocally alter a property of the lateral acoustic wave in thepiezoelectric layer. One of (i) the plurality of electrodes or (ii) theat least one loading material is arranged in the slow wave propagationlayer, and the other one of (i) the plurality of electrodes or (ii) theat least one loading material is arranged over the piezoelectric layer.In certain embodiments, the plurality of electrodes is arranged in theslow wave propagation layer, and the at least one loading material isarranged over the piezoelectric layer. In other embodiments, theplurality of electrodes is arranged over the piezoelectric layer, andthe at least one loading material is arranged in the slow wavepropagation layer.

In certain embodiments, a micro-electrical-mechanical system (MEMS)guided wave device includes: a single crystal piezoelectric layer; aslow wave propagation layer disposed below the single crystalpiezoelectric layer; at least one embedded electrode arranged in theslow wave propagation layer; at least one top side electrode arrangedover the single crystal piezoelectric layer; and a guided waveconfinement structure arranged proximate to the slow wave propagationlayer; wherein either (i) the at least one embedded electrode or (ii)the at least one top side electrode, includes a plurality of electrodesconfigured for transduction of at least one lateral acoustic wave in thesingle crystal piezoelectric layer; wherein the guided wave confinementstructure is configured to confine the at least one lateral acousticwave in the single crystal piezoelectric layer and the slow wavepropagation layer; and wherein at least one region of the single crystalpiezoelectric layer is sandwiched between an embedded electrode of theat least one embedded electrode and a top side electrode of the at leastone top side electrode. In certain embodiments, the plurality ofelectrodes configured for transduction of the at least one lateralacoustic wave in the single crystal piezoelectric layer includes aninterdigital transducer (IDT) including two groups of electrodes ofopposing polarity.

In certain embodiments, one or more regions of a piezoelectric layer maybe sandwiched between one or more embedded electrodes and one or moretop side electrodes. Such piezoelectric layer may preferably includesingle crystal piezoelectric material. The resulting structure mayprovide various utilities. In one implementation, at least oneconductive trace or via may be defined through the single crystalpiezoelectric layer to provide ohmic contact to an embedded electrode,thereby providing an electrically conductive path including the embeddedelectrode. In another implementation, a region of a single crystalpiezoelectric layer may be sandwiched between an embedded electrode anda top side electrode to form at least one capacitive element. In anotherimplementation, an embedded electrode in combination with a top sideelectrode form an output electrode, and are used together with first andsecond groups of input electrodes to interact with a single crystalpiezoelectric layer to provide non-linear elastic convolver utility.

In certain embodiments, one or more regions of a single crystalpiezoelectric layer are sandwiched between an embedded electrode and atop side electrode to yield one or more capacitive elements. In certainembodiments, patterning and/or trimming of top side contact material(e.g., metal) may be employed to realize different capacitance values.In situations in which single crystal piezoelectric materials such aslithium niobate or lithium tantalate are used, the very high dielectricconstant and the small thickness (e.g., ranging from sub-micron to a fewmicrons) renders the capacitive sensing very efficient, thereby enablingsize efficient capacitors integrated with MEMS guided wave (e.g.,resonator) devices. In certain embodiments, one or more capacitiveelements may be optionally arranged in series or arranged in parallelwith at least some embedded electrodes that are configured fortransduction of a lateral acoustic wave in the piezoelectric layer. Incertain embodiments, one or more capacitive elements may be used fornon-contact sensing of signals.

In certain embodiments, at least one capacitive element may beelectrically coupled in series, or in parallel, with at least oneembedded electrode. In certain embodiments, one or more capacitiveelements may include bus bars of an IDT. In certain embodiments, atleast two top side electrodes are arranged to interact with at least twoembedded electrodes to provide non-contact sensing utility. In a sensorapplication utilizing capacitive elements, it may be desirable to sensea signal without disturbing an embedded structure, such as in asituation where the environment being sensed may be detrimental toburied electrode material. In such a case, top sensing electrodes may bemade of a non-corrosive metal such as gold, platinum, nickel, or thelike. In certain embodiments, the at least one top side electrode isnon-coincident with an active region of the piezoelectric layer.

In certain embodiments, a method of fabricating a MEMS guided wavedevice including one or more embedded electrodes and one or more topside electrodes includes multiple steps. Such steps may includeproviding at least one embedded electrode in a slow wave propagationlayer; arranging a single crystal piezoelectric layer over the slow wavepropagation layer; and providing at least one top side electrodearranged over the single crystal piezoelectric layer. Either (i) the atleast one embedded electrode or (ii) the at least one top side electrodeincludes a plurality of electrodes configured for transduction of alateral acoustic wave in the piezoelectric layer. The guided waveconfinement structure is configured to confine the lateral acoustic wavein the single crystal piezoelectric layer and the slow wave propagationlayer, and one or more regions of the single crystal piezoelectric layerare sandwiched between (i) one or more embedded electrodes of the atleast one embedded electrode and (ii) one or more top side electrodes ofthe at least one top side electrode. In certain embodiments, providing aplurality of embedded electrodes in the slow wave propagation layerincludes defining a plurality of recesses in the slow wave propagationlayer, and depositing electrode material in the plurality of recesses.In certain embodiments, the piezoelectric layer may be locally thinnedto define a first thickness region and a second thickness region,wherein a thickness of the first thickness region differs from athickness of the second thickness region.

FIG. 13A is a top plan view, and FIG. 13B is a cross-sectional view, ofa MEMS guided wave device suitable for providing non-contact capacitivesensing utility. Transduction of an input stimulus to the acoustic wavedevice allows the acoustic wave device to respond by changing itsresonant frequency. The device includes a piezoelectric layer 30overlying a slow wave propagation layer 26 having first and secondelectrodes (that are electrically isolated relative to one another)embedded therein to form an IDT 58 composed of bus bars 52 and fingers54, with the IDT 58 being in contact with the piezoelectric layer 30.The slow wave propagation layer 26 overlies a guided wave confinementstructure 28 (e.g., a fast wave propagation layer or Bragg mirror).First and second top side electrodes 66, 66A are formed over the exposedupper surface 30′ of the piezoelectric layer 30 and generally overlapthe bus bars 52 (which also embody electrodes) embedded in the slow wavepropagation layer 26. As shown in FIG. 13B, two portions of thepiezoelectric layer 30 are sandwiched between top sideelectrode/embedded electrode pairs 66, 52 and 66A, 52, thereby formingcapacitor elements. The resulting MEMS guided wave device is suitablefor providing non-contact capacitive sensing utility. In certainembodiments, multiple capacitors may be integrated in a MEMS guided wavedevice, wherein such capacitors may be connected in series, in parallel,or in a series-parallel configuration relative to one another, and/ormay be connected in series, in parallel, or in a series-parallelconfiguration relative to one or more IDTs or other electrodes.

In certain embodiments, a capacitor may be arranged in series with anIDT (or other electrodes). FIG. 14A is a top plan view, and FIGS. 14Band 14C are cross-sectional views, of a MEMS guided wave deviceincluding embedded electrodes that is suitable for producing an IDT inseries with a capacitor. The device includes a piezoelectric layer 30overlying a slow wave propagation layer 26 having electrodes embeddedtherein to form an IDT 58 composed of bus bars 52 and fingers 54, withthe IDT 58 being in contact with the piezoelectric layer 30. The slowwave propagation layer 26 overlies a guided wave confinement structure28 (e.g., a fast wave propagation layer or Bragg mirror). A top sideelectrode 66 is arranged over the exposed top surface 30′ of thepiezoelectric layer 30 and generally overlaps one bus bar 52 embedded inthe slow wave propagation layer 26. An opening 68 is defined (e.g., viaetching) through the piezoelectric layer 30 to expose an embeddedelectrode in the form of a bus bar 52, as shown in FIG. 14B. Suchopening 68 is preferably defined prior to application of the top sideelectrode 66. Referring to FIG. 14C, a conductive contact material 70may be deposited in the opening 68 to form a raised ohmic contactextending through the piezoelectric layer 30, thereby providing anexternally accessible conductive connection to the bus bar 52 embeddedin the slow wave propagation layer 26. As shown at left in FIG. 14C, aportion of the piezoelectric layer 30 is sandwiched between one top sideelectrode 66 and a recessed contact (i.e., bus bar 52) to form acapacitor. Such capacitor is arranged in series with the IDT 58 (asshown in FIG. 14A).

Although FIG. 14A shows the top side electrode 66 as overlappingsubstantially the entirety of one bus bar 52, it is to be appreciatedthat in alternative embodiments, one or more top side electrodes may bearranged to overlap only a portion of a bus bar, or to overlap one ormore embedded electrodes that are laterally displaced from a bus bar.Providing series and/or parallel capacitive elements that are displacedfrom (outside) a bus bar area may avoid potentially detrimentalasymmetric loading of a symmetric waveguide structure.

FIG. 14D is a cross-sectional view of an alternative MEMS guided wavedevice similar to that of FIG. 14C, but in which electrical conductionwith embedded electrodes is provided without formation of any capacitiveelement formed by one or more sandwiched portions of the piezoelectriclayer. The device includes a piezoelectric layer 30 overlying a slowwave propagation layer 26 having electrodes embedded therein to form anIDT composed of bus bars 52 and fingers 54, with the IDT being incontact with the piezoelectric layer 30. The slow wave propagation layer26 overlies a guided wave confinement structure 28. Conductive contactmaterials 70A, 70B are deposited in openings defined through thepiezoelectric layer 30, to form raised ohmic contacts to the bus bars 52embedded in the slow wave propagation layer 26. As will be apparent toone skilled in the art, ohmic contacts may be made with bus bars orother electrodes at any suitable location, either within an activeregion or outside of an active region of a MEMS guided wave device.

In certain embodiments, a capacitor may be arranged in parallel with anIDT (or other electrodes). FIG. 15A is a top plan view, and FIGS. 15Band 15C are cross-sectional views, of a MEMS guided wave deviceincluding embedded electrodes, two capacitors in series with an IDT, andanother capacitor in parallel with the IDT. The device includes apiezoelectric layer 30 overlying a slow wave propagation layer 26 havingelectrodes embedded therein to form an IDT 58 composed of bus bars 52and fingers 54, with the IDT 58 being in contact with the piezoelectriclayer 30. An additional embedded electrode 72 is arranged in parallelwith the IDT 58. The slow wave propagation layer 26 overlies a guidedwave confinement structure 28 (e.g., a fast wave propagation layer orBragg mirror). First and second top side electrodes 66, 66A are arrangedover the exposed top surface 30′ of the piezoelectric layer 30. Thefirst top side electrode 66 overlaps the piezoelectric layer 30 and onebus bar 52 embedded in the slow wave propagation layer 26 to form acapacitor in series with the IDT 58, and further extends to an ohmic(conductive) contact material 70 that extends through the piezoelectriclayer 30 into contact with the additional embedded electrode 72. Thesecond top side electrode 66A overlaps the piezoelectric layer 30 andanother bus bar 52 embedded in the slow wave propagation layer 26 toform another capacitor in series with the IDT 58, and includes anL-shaped extension 66A′ that covers the piezoelectric layer 30 andanother portion of the additional embedded electrode 72. As shown inFIG. 15C, a portion of the piezoelectric layer 30 is sandwiched betweena (rightmost) portion of the additional embedded electrode 72 and theL-shaped extension 66A′ of the second top side electrode 66A to form acapacitor coupled in parallel with the IDT 58 and the two capacitors inseries with the IDT (referenced above).

As noted previously, in certain embodiments, an embedded electrode incombination with a top side electrode may form an output electrode to beused together with first and second groups of input electrodes tointeract with a single crystal piezoelectric layer to provide non-linearelastic convolver utility. An acoustic wave convolver may be usedbetween an incoming signal message bit and a locally providedtime-reversed reference replica of coding applied to the message signal.Acoustic convolvers are useful in spread-spectrum wireless applicationsfor packet-data and packet-voice communications, and are also suited toreduce multipath interference. Examples of structures suitable forproviding non-linear elastic convolver utility are illustrated in FIGS.24A, 24B, 25A, and 25B.

FIG. 24A is a cross-sectional view of a MEMS guided wave deviceincluding an exposed piezoelectric layer 30 (preferably consisting ofsingle crystal piezoelectric material) having internal electrodes 38,40, 38A, 40A embedded in a slow wave propagation layer 26 underlying thepiezoelectric layer 30, and including a guided wave confinementstructure 28 underlying the slow wave propagation layer 26. A firstgroup of electrodes 38, 40 of opposing polarity form a firstinterdigital transducer, and a second group of electrodes 38A, 40A ofopposing polarity form a second interdigital transducer that islaterally spaced from the first interdigital transducer, with the firstand second interdigital transducers serving as input electrode groups.An external (or top side) plate electrode 122 is arranged in contactwith an exposed upper surface 30′ of the piezoelectric layer 30, and anembedded plate electrode 130 is arranged in contact with a lower surface30″ of the piezoelectric layer 30 and embedded within the slow wavepropagation layer 26 to sandwich a central portion of the piezoelectriclayer 30, with the sandwich structure forming an output electrode. Theoutput electrode is positioned laterally between the first and secondinterdigital transducers, whereby the first interdigital transducer, thesecond interdigital transducer, and the output electrode are configuredto interact with the piezoelectric layer 30 to provide non-linearelastic convolver utility. Such a device utilizes nonlinearity of thepiezoelectric medium to perform signal processing.

In operation of the device of FIG. 24A, a coded input signal having afrequency f1 is supplied to alternating electrodes 38, 40 of the firstIDT, a coded input signal having a frequency f1 is supplied toalternating electrodes 38A, 40A of the second IDT, and acoustic wavesgenerated in the piezoelectric layer 30 by the IDTs are received by theoutput electrode, providing an autocorrelated output signal.

FIG. 24B is a cross-sectional view of a MEMS guided wave device similarto the device of FIG. 24A, but including externally arranged (or topside) input electrodes 38, 40, 38A, 40A arranged over an exposed uppersurface 30′ of a piezoelectric layer 30. The piezoelectric layer 30(preferably consisting of single crystal piezoelectric material)overlies a slow wave propagation layer 26, which overlies a guided waveconfinement structure 28. A first group of top side electrodes 38, 40 ofopposing polarity form a first interdigital transducer, and a secondgroup of top side electrodes 38A, 40A of opposing polarity form a secondinterdigital transducer that is laterally spaced from the firstinterdigital transducer, with the first and second interdigitaltransducers serving as input electrode groups. An external (or top side)plate electrode 122 is arranged in contact with an exposed upper surface30′ of the piezoelectric layer 30, and an embedded plate electrode 130is arranged in contact with a lower surface 30″ of the piezoelectriclayer 30 that is embedded within the slow wave propagation layer 26 tosandwich a central portion of the piezoelectric layer 30, with thesandwich structure forming an output electrode. The output electrode ispositioned laterally between the first and second interdigitaltransducers, whereby the first interdigital transducer, the secondinterdigital transducer, and the output electrode are configured tointeract with the piezoelectric layer 30 to provide non-linear elasticconvolver utility. Operation of the device of FIG. 24B is substantiallysimilar to the above-described operation of the device of FIG. 24A.

In certain embodiments, non-linear elastic convolver utility may beprovided with devices utilizing a guided wave confinement structureincluding a cavity or recess arranged below a piezoelectric layer, suchthat a portion of the piezoelectric layer is suspended. Such a devicemay include either internal or top side interdigital transducers asinput electrodes in combination with an output electrode that includesan external (or top side) plate electrode and an internal (or embedded)plate electrode arranged to sandwich a central portion of apiezoelectric layer. In certain embodiments, an enclosed cavity orrecess may be produced by selective removal of material from one or moreprecursor layers to form a recess, addition of sacrificial material tothe recess, and subsequent removal of the sacrificial material (e.g.,using a liquid etchant) to yield a partially or fully enclosed cavity orrecess. The use of sacrificial material during fabrication, followed byselective removal of sacrificial material, beneficially permitsmicrostructures to be formed without distortion that would otherwise belikely during processing steps such as planarization/polishing,interlayer bonding, or the like.

FIG. 25A is a cross-sectional view of a MEMS guided wave deviceincluding a piezoelectric layer 30 (preferably consisting of singlecrystal piezoelectric material) arranged over a cavity or recess 82defined in a slow wave propagation layer 26 underlying the piezoelectriclayer 30. A substrate 28A (optionally embodying fast wave propagationmaterial) underlies the slow wave propagation layer 26. A portion of thepiezoelectric layer 30 is suspended over the cavity or recess 82, whichserves as a guided wave confinement structure to confine a lateralacoustic wave in the piezoelectric layer 30. Internal electrodes 38, 40,38A, 40A are supported by a lower surface 30″ of the suspended portionof the piezoelectric layer 30. A first group of electrodes 38, 40 ofopposing polarity form a first interdigital transducer, and a secondgroup of electrodes 38A, 40A of opposing polarity form a secondinterdigital transducer that is laterally spaced from the firstinterdigital transducer, with the first and second interdigitaltransducers serving as input electrode groups. An external (or top side)plate electrode 122 is arranged in contact with an upper surface 30′ ofthe piezoelectric layer 30, and an embedded plate electrode 130 isarranged in contact with the lower surface 30″ of the suspended portionof the piezoelectric layer 30, with the sandwich structure forming anoutput electrode. The output electrode is positioned laterally betweenthe first and second interdigital transducers, whereby the firstinterdigital transducer, the second interdigital transducer, and theoutput electrode are configured to interact with the piezoelectric layer30 to provide non-linear elastic convolver utility. Such a deviceutilizes nonlinearity of the piezoelectric medium to perform signalprocessing. Operation of the device of FIG. 25A is substantially similarto the above-described operation of the device of FIG. 24A.

FIG. 25B is a cross-sectional view of a MEMS guided wave device similarto the device of FIG. 25A, but including external (or top side) inputelectrodes 38, 40, 38A, 40A arranged over an upper surface 30′ of thesuspended portion of a piezoelectric layer 30. The piezoelectric layer30 (preferably consisting of single crystal piezoelectric material) isarranged over a cavity or recess 82 defined in a slow wave propagationlayer 26 underlying the piezoelectric layer 30, and a substrate 28A(optionally including fast wave propagation material) underlies the slowwave propagation layer 26. A portion of the piezoelectric layer 30 issuspended over the cavity or recess 82, which serves as a guided waveconfinement structure. External (or top side) input electrodes 38, 40,38A, 40A are supported by the upper surface 30′ of the suspended portionof the piezoelectric layer 30. A first group of electrodes 38, 40 ofopposing polarity form a first interdigital transducer, and a secondgroup of electrodes 38A, 40A of opposing polarity form a secondinterdigital transducer that is laterally spaced from the firstinterdigital transducer, with the first and second interdigitaltransducers serving as input electrode groups. An external (or top side)plate electrode 122 is arranged in contact with the upper surface 30′ ofthe piezoelectric layer 30, and an embedded plate electrode 130 isarranged in contact with a lower surface 30″ of the suspended portion ofthe piezoelectric layer 30, with the sandwich structure forming anoutput electrode. The output electrode is positioned laterally betweenthe first and second interdigital transducers, whereby the firstinterdigital transducer, the second interdigital transducer, and theoutput electrode are configured to interact with the piezoelectric layer30 to provide non-linear elastic convolver utility. Operation of thedevice of FIG. 25B is substantially similar to the above-describedoperation of the device of FIG. 24A.

Consistent with FIGS. 25A and 25B, in certain embodiments amicro-electrical-mechanical system (MEMS) guided wave device includes: asingle crystal piezoelectric layer; a substrate; a cavity or recessarranged between the substrate and a suspended portion of the singlecrystal piezoelectric layer; a first group of input electrodesconfigured for transduction of a first lateral acoustic wave in thesuspended portion of the single crystal piezoelectric layer; a secondgroup of input electrodes configured for transduction of a secondlateral acoustic wave in the suspended portion of the single crystalpiezoelectric layer; and an output electrode including a first platearranged over the suspended portion of the single crystal piezoelectriclayer and a second plate arranged under the suspended portion of thesingle crystal piezoelectric layer; wherein the first group of inputelectrodes, the second group of input electrodes, and the outputelectrode are configured to interact with the single crystalpiezoelectric layer to provide non-linear elastic convolver utility. Incertain embodiments, at least one sensing material is arranged over atleast a portion of the piezoelectric layer, wherein at least oneproperty of at least one sensing material is configured to change inexposure to an environment proximate to the at least one sensingmaterial, and at least one wave propagation property of thepiezoelectric layer may be altered in response to such change. Inpreferred embodiments, at least one sensing material is provided indirect contact with a piezoelectric layer to promote increasedsensitivity due to direct interaction between an acoustic wave and theat least one sensing material; however, in alternative embodiments, oneor more intervening layers (e.g., interface and/or adhesion promotinglayers) may be provided between a piezoelectric layer and one or moresensing materials.

In certain embodiments, a micro-electrical-mechanical system (MEMS)guided wave device includes a piezoelectric layer; a plurality ofelectrodes disposed below the piezoelectric layer and configured fortransduction of at least one lateral acoustic wave in the piezoelectriclayer; a guided wave confinement structure configured to confine the atleast one lateral acoustic wave in the piezoelectric layer; and at leastone sensing material arranged over at least a portion of thepiezoelectric layer; wherein at least one property of the at least onesensing material is configured to change in exposure to an environmentproximate to the at least one sensing material.

In certain embodiments, a MEMS guided wave device includes a delay linewith an input IDT or PPT, an output IDT or PPT, and a delay path(including a piezoelectric layer) between the input and output IDTs orPPTs, wherein at least one sensing material is arranged to modify apropagation property of a wave traveling in or through the delay path.In certain embodiments, a MEMS guided wave device includes a resonatorwith first and second IDTs or PPTs arranged between reflector gratings,wherein at least one sensing material is arranged over at least aportion of the resonator and arranged to alter a signal transduced by atleast one of the IDTs or PPTs.

MEMS guided wave devices are sensitive to mechanical and electricalproperties occurring on their surfaces. With respect to mechanicalproperties, such devices are sensitive to mass loading and viscoelasticchanges like stiffening and softening. For example, mass loading as atarget material is absorbed or otherwise bound to the surface of asensing material will result in a decrease in oscillation frequency. Asanother example, changes in a sensing film as a material target diffusesinto the bulk of the sensing film can result in elastic stiffening orsoftening. Elastic stiffening will result in an increase in oscillationfrequency, whereas elastic softening or swelling of a sensing film willresult in a decrease in oscillation frequency. With respect toelectrical properties, such devices may be sensitive to properties thatinteract with an electrical field coupled to the propagating acousticwave. For example, conductivity changes in a sensing film as it isexposed to a concentration of target material can result in an increaseor decrease in oscillation frequency depending on whether the targetmaterial causes the conductivity of a sensing material to increase ordecrease. The preceding effects may be termed electro-acousticinteractions. In certain embodiments, properties of sensing materialsthat may be subject to change upon exposure to target materials mayinclude adsorption, reaction rates, curing, phase change, reactivity, orthe like.

In certain embodiments, at least one sensing material may be arranged tointeract with a gas. In other embodiments, at least one sensing materialmay be arranged to interact with a liquid.

In certain embodiments, at least one property of at least one sensingmaterial is configured to change responsive to a change of presence orconcentration of one or more chemical species in an environmentproximate to the at least one sensing material. In certain embodiments,a sensing material may comprise a chemical-specific sensing film. Uponexposure of the sensing film to a chemical species, mechanical andelectrical perturbations in the sensing film will cause a correspondingchange in the oscillation frequency of the acoustic wave transduced inan underlying piezoelectric layer. Examples of sensing films may includemetals, metal oxides, metal nitrides, or polymers. In certainembodiments, at least one sensing material is configured to undergo areversible change in exposure to one or more chemical species; in otherembodiments, at least one sensing material may be configured to undergoan irreversible change in exposure to one or more chemical species.

In certain embodiments, at least one property of at least one sensingmaterial is configured to change responsive to a change of presence orconcentration of one or more biological species in an environmentproximate to the at least one sensing material. For example, at leastone sensing material may be configured for specific binding to one ormore biological moieties. In certain embodiments, at least one sensingmaterial may be configured to interact with pathogens, cells, cellconstituents, bacteria, proteins, ligands, and/or antibodies. In certainembodiments, a sensing material may comprise a biologicalmoiety-specific material, such as antigens, bacterial biofilms, cellcultures, or the like.

In certain embodiments, at least one property of at least one sensingmaterial is configured to change responsive to a change of presence orstrength of a magnetic field and/or an electrical field in anenvironment proximate to the sensing material. For example, a sensingmaterial may include a magnetically responsive material (e.g., InSb,InAs, NiSb, ferromagnetic materials, and/or antiferromagneticmaterials). In certain embodiments, at least one property of at leastone sensing material is configured to change responsive to a change oftemperature in an environment proximate to the sensing material. Forexample, a sensing material may include a shape-memory material (e.g., ametal alloy such as NiTi alloy, or a shape memory polymer such asprogrammed poly(methyl methacrylate) (PMMA), ethylene vinyl acetate(EVA) or polylactide (PLA)) or a pyrometric element (e.g., lithiumtantalate). In certain embodiments, the at least one sensing materialmay comprise a semiconductor material.

In certain embodiments, at least one property of at least one sensingmaterial is configured to change responsive to stress applied to asensing material. In certain embodiments, at least one property of atleast one sensing material is configured to change responsive toacceleration applied to a sensing material. In certain embodiments, atleast one property of at least one sensing material is configured tochange responsive to change of humidity experienced by the sensingmaterial.

In certain embodiments, at least one property of at least one sensingmaterial is configured to change responsive to receipt of radiation. Incertain embodiments, at least one sensing material may be responsive toinfrared radiation, visible light, ultraviolet radiation, or any otherspectral range. For example, a polymer-containing sensing material mayhave viscoelastic properties that are subject to change in exposure toradiation, such as by crosslinking. Other physical interactions betweenradiation and at least one sensing material may be employed.

In certain embodiments, at least one sensing material is coated over theentirety (or substantially the entirety) of a piezoelectric layer. Inother embodiments, one or more sensing materials may be selectivelyapplied and/or patterned over different regions of a piezoelectriclayer. In certain embodiments, multiple sensing materials may beprovided over different regions of a piezoelectric layer.

In certain embodiments, a MEMS guided wave device includes a sensordelay line with a first pair of IDTs or PPTs separated by a first delaypath in a piezoelectric material coated or otherwise covered with atleast one sensing material, and includes a reference delay line with asecond pair of IDTs or PPTs separated by a second delay path in thepiezoelectric material that is not coated or otherwise covered with asensing material.

In certain embodiments, a MEMS guided wave device includes a sensorresonator with a first pair of IDTs or PPTs arranged between reflectorgratings with at least a portion of a piezoelectric material of thesensor resonator coated or otherwise covered with at least one sensingmaterial, and includes a reference resonator with a second pair of IDTsor PPTs arranged between reflector gratings with the reference resonatorbeing devoid of any coating or covering of sensing material.

FIG. 16A is a top plan view, and FIG. 16B is a cross-sectional view, ofa MEMS guided wave device including a piezoelectric layer 30 arrangedover electrodes (i.e., interdigital transducers 58-1, 58A-1, 58-2, 58A-2and reflector gratings 50-1, 50-2 forming first and second resonators74, 76) embedded in a slow wave propagation layer 26 underlying thepiezoelectric layer 30. A guided wave confinement structure 28 furtherunderlies the slow wave propagation layer 26. Sensing material 80-1 isarranged over an upper surface 30′ of the piezoelectric layer 30 overthe first resonator 74 (including the first pair of IDTs 58-1, 58A-1 andreflector gratings 50-1) to provide sensing utility. The first pair ofIDTs 58-1, 58A-1 and the reflector gratings 50-1 include bus bars 52-1and fingers 54-1. The second resonator 76 includes a second pair of IDTs58-2, 58A-2 arranged between a second pair of reflector gratings 50-2,with the second resonator 76 being devoid of any sensing material, toserve as a reference against which signals generated by the firstresonator 74 can be compared. The second pair of IDTs 58-2, 58A-2 andthe reflector gratings 50-2 include bus bars 52-2 and fingers 54-2. Inthe first pair of IDTs 58-1, 58A-1, one IDT 58-1 includes one group ofalternating electrodes 38, 40, and the other IDT 58A-1 includes anothergroup of alternating electrodes 38A, 40A. In operation of the MEMSguided wave device of FIGS. 16A and 16B, voltage is applied across busbars 52-1, 52-2 of a transmitting IDT 58-1, 58-2 in each of the firstresonator 74 and the second resonator 76. An acoustic wave transduced byeach transmitting IDT 58-1, 58-2 is propagated through the piezoelectriclayer 30 to a corresponding receiving IDT 58A-1, 58A-2. The firstresonator 74 includes the sensing material 80-1, whereas the secondresonator 76 lacks any sensing material. By comparing signals receivedby the receiving IDT 58A-1 of the first resonator 74 and the receivingIDT 58A-2 of the second resonator 76. any changes in one or more wavepropagation properties between the resonators 74, 76 may be used todetect interaction between the sensing material 80-1 and a surroundingenvironment.

FIG. 17A is a top plan view, and FIG. 17B is a cross-sectional view, ofa MEMS guided wave device including a piezoelectric layer 30 arrangedover electrodes (i.e., interdigital transducers 58-1, 58A-1, 58-2, 58A-2forming first and second delay lines 74A, 76A) embedded in a slow wavepropagation layer 26 underlying the piezoelectric layer 30. A guidedwave confinement structure 28 (e.g., a fast wave propagation layer orBragg mirror) further underlies the slow wave propagation layer 26. Asensor delay line 74A includes sensing material 80-1 arranged over theupper surface 30′ of the piezoelectric layer 30 between a first pair ofIDTs 58-1, 58A-1 to provide sensing utility. The sensing material 80-1overlies the piezoelectric layer 30 solely in an area between the firstpair of IDTs 58-1, 58A-1, but in alternative embodiments may be extendedto additionally cover the first pair of IDTs 58-1, 58A-1. The first pairof IDTs 58-1, 58A-1 includes bus bars 52-1 and fingers 54-1. A referencedelay line 76A includes a second pair of IDTs 58-2, 58A-2, with the areatherebetween being devoid of any sensing material, to serve as areference against which signals generated by the sensor delay line 74Acan be compared. The second pair of IDTs 58-2, 58A-2 includes bus bars52-2 and fingers 54-2. In the first pair of IDTs 58-1, 58A-1, one IDT58-1 includes one group of alternating electrodes 38, 40, and the otherIDT 58A-1 includes another group of alternating electrodes 38A, 40A, asshown in FIG. 17B.

In certain embodiments, multiple sensing materials and multiple sensorresonators may be provided in a single MEMS guided wave device. FIG. 18illustrates a MEMS guided wave device including a piezoelectric layerhaving an exposed upper surface 30′ and being arranged over electrodes(i.e., interdigital transducers 58-1, 58A-1, 58-2, 58A-2, 58-3, 58A-3,58-4, 58A-4 and reflector gratings 50-1, 50-2, 50-3, 50-4 formingresonators 74-1 to 74-4) embedded in a slow wave propagation layerunderlying the piezoelectric layer. First through fourth resonators74-1, 74-2, 74-3, 74-4 each include a different sensing material 80-1,80-2, 80-3, 80-4 arranged over the upper surface 30′ of thepiezoelectric layer between respective pairs of IDTs 58-1, 58A-1, 58-2,58A-2, 58-3, 58A-3, 58-4, 58A-4 to provide sensing utility. Each pair ofIDTs 58-1, 58A-1, 58-2, 58A-2, 58-3, 58A-3, 58-4, 58A-4 is arrangedbetween a pair of laterally spaced reflector gratings 50-1, 50-2, 50-3,50-4. Within each resonator 74-1, 74-2, 74-3, 74-4, the IDTs 58-1,58A-1, 58-2, 58A-2, 58-3, 58A-3, 58-4, 58A-4 and the reflector gratings50-1, 50-2, 50-3, 50-4 include bus bars 52-1, 52-2, 52-3, 52-4 andfingers 54-1, 54-2, 54-3, 54-4. In operation of the MEMS guided wavedevice of FIG. 18, voltage is applied across bus bars of a transmittingIDT 58-1, 58-2, 58-3, 58-4 in each resonator 74-1, 74-2, 74-3, 74-4. Anacoustic wave transduced by each transmitting IDT 58-1, 58-2, 58-3, 58-4is propagated through the piezoelectric layer 30 to a correspondingreceiving IDT 58A-1, 58A-2, 58A-3, 58A-4, and changes in wavepropagation properties for each resonator 74-1, 74-2, 74-3, 74-4 may beused to detect interaction between the sensing material 80-1, 80-2,80-3, 80-4 and a surrounding environment.

In certain embodiments, MEMS guided wave devices may be configured todetect changes in pressure, such as by providing a portion of apiezoelectric layer that is suspended over a sealed cavity or recess,whereby deflection of the suspended portion in exposure to pressurechanges may result in detectable alteration of a wave propagationproperty of the piezoelectric layer. In certain embodiments, a MEMSguided wave device includes: a piezoelectric layer; a plurality ofelectrodes disposed below the piezoelectric layer and configured fortransduction of at least one lateral acoustic wave in the piezoelectriclayer; and at least one underlying layer arranged proximate to thepiezoelectric layer and defining a sealed cavity or recess bounded by asuspended portion of the piezoelectric layer; wherein a wave propagationproperty of the piezoelectric layer is configured to change in responseto exposure of the piezoelectric layer to a change in pressure of anenvironment proximate to the piezoelectric layer. In certainembodiments, the at least one underlying layer comprises a slow wavepropagation layer disposed below a portion of the piezoelectric layer,wherein the plurality of electrodes is arranged in the slow wavepropagation layer. In certain embodiments, the plurality of electrodesis supported by the suspended portion of the piezoelectric layer. Incertain embodiments, the underlying layer comprises a guided waveconfinement structure configured to confine the at least one lateralacoustic wave in the piezoelectric layer and (if present) the slow wavepropagation layer. In certain embodiments, at least one functional layer(e.g., a sensing material, a temperature compensation material, a slowwave propagation material, a semiconducting material, and/or layer(s)conferring mixed domain signal processing utility) may be arranged overat least a portion of the piezoelectric layer, including the suspendedportion thereof.

FIG. 19A is a top view, and FIG. 19B is a cross-sectional view, of aMEMS guided wave device configured to detect changes in pressure. TheMEMS guided wave device includes an piezoelectric layer 30 (preferablyconsisting of single crystal piezoelectric material) having an exposedupper surface 30′ and being arranged over a cavity or recess 82 definedin a slow wave propagation layer or sacrificial material layerunderlying the piezoelectric layer 30 to define a suspended portion 88of the piezoelectric layer 30. An optional intermediate layer 27 (whichmay serve as an etch stop, bonding, or interface layer) and anunderlying substrate 28A are provided below the cavity or recess 82,which serves as a guided wave confinement structure to confine a lateralacoustic wave in the piezoelectric layer 30. Internal electrodes (i.e.,interdigital transducers 58, 58A and reflector gratings 50 forming aresonator 84) are supported by a lower surface 30″ of the suspendedportion 88 of the piezoelectric layer 30. A first group of electrodes38, 40 of opposing polarity form a first interdigital transducer 58, anda second group of electrodes 38A, 40A of opposing polarity form a secondinterdigital transducer 58A that is laterally spaced from the firstinterdigital transducer. In certain embodiments, optional verticalopenings 86 (shown in FIG. 19A) may be provided during manufacture toenable an etchant to be supplied through the piezoelectric layer 30 toan underlying (sacrificial or slow wave propagation layer) to define thecavity or recess 82, while the intermediate layer 27 may preventinteraction between etchant and the substrate 28A; however, suchopenings 86 are preferably sealed (e.g., with epoxy or another suitablematerial) after fabrication to avoid equalization of pressure betweenthe cavity or recess 82 and a surrounding environment (which wouldinterfere with pressure detection). In less preferred embodiments, thecavity or recess 82 may be defined in one or more layers (e.g., viaetching) before attachment of the piezoelectric layer 30 (e.g., viadirect bonding). The IDTs 58, 58A and the reflector gratings 50 includebus bars 52 and fingers 54. In operation of the MEMS guided wave deviceof FIGS. 19A and 19B, voltage is applied across bus bars 52 of atransmitting IDT 58, and an acoustic wave is propagated through thesuspended portion 88 of the piezoelectric layer 30 to a receiving IDT58A. When the suspended portion 88 of the piezoelectric layer 30 isdeflected (e.g., in exposure to a change in pressure), changes in wavepropagation properties between the paired IDTs 58, 58A due to suchdeflection may be used to detect changes in pressure in a surroundingenvironment.

FIG. 20A is a top view, and FIG. 20B is a cross-sectional view, ofanother MEMS guided wave device configured to detect changes inpressure, but having a smaller cavity or recess 82A underlying thepiezoelectric layer 30. The MEMS guided wave device includes apiezoelectric layer 30 having an exposed upper surface 30′, withelectrodes (including interdigital transducers and gratings) embedded ina slow wave propagation layer 26 underlying the piezoelectric layer 30.A guided wave confinement structure 28 underlies the slow wavepropagation layer 26, with an optional intermediate layer 27 (which mayserve as an etch stop, bonding, or interface layer) being arrangedtherebetween. As shown in FIG. 20B, a suspended portion 88 of thepiezoelectric layer 30 is arranged over a cavity or recess 82A definedin the slow wave propagation layer 26. In certain embodiments, optionalvertical openings 86 (shown in FIG. 20A) may be provided duringmanufacture to enable an etchant to be supplied through thepiezoelectric layer 30 to the slow wave propagation layer 26 to definethe cavity or recess 82A while the intermediate layer 27 may preventinteraction between etchant and the guided wave confinement structure28; however, such openings 86 are preferably sealed after fabrication toavoid equalization of pressure between the cavity or recess 82A and asurrounding environment. In less preferred embodiments, the cavity orrecess 82A may be defined in the slow wave propagation layer 26 (e.g.,via etching) before attachment of the piezoelectric layer 30 (e.g., viadirect bonding) over the slow wave propagation layer 26. Thepiezoelectric layer 30 is arranged over electrodes includinginterdigital transducers 58, 58A and reflector gratings 50 embedded inan upper portion of the slow wave propagation layer 26. The IDTs 58, 58Aand the reflector gratings 50 include bus bars 52 and fingers 54. In theIDTs 58, 58A, one IDT 58 includes a first group of alternatingelectrodes 38, 40, and the other IDT 58A-1 includes a second group ofalternating electrodes 38A, 40A. The IDTs 58, 58A and piezoelectriclayer 30 form a resonator 84. In operation of the MEMS guided wavedevice of FIGS. 20A and 20B, voltage is applied across bus bars 52 of atransmitting IDT 58, and an acoustic wave is propagated through thesuspended portion 88 of the piezoelectric layer 30 to a receiving IDT58A. When the suspended portion 88 of the piezoelectric layer 30 isdeflected (e.g., in exposure to a change in pressure), changes in wavepropagation properties between the paired IDTs 58, 58A due to suchdeflection may be used to detect changes in pressure in a surroundingenvironment.

In certain embodiments, one or more functional materials and/or sensingmaterials may be arranged over a suspended portion of a piezoelectriclayer of a MEMS guided wave device. Such an arrangement may provideenhanced detection sensitivity.

FIG. 21A is a top plan view, and FIG. 21B is a partial cross-sectionalview, of a MEMS guided wave device bearing similarity to the device ofFIGS. 19A and 19B, but modified to include a functional material and/orsensing material 80 arranged over the upper surface 30′ of the suspendedportion 88 of the piezoelectric layer 30, omitting reflector gratings,and including a delay line 84′ instead of a resonator. The MEMS guidedwave device includes an exposed piezoelectric layer 30 (preferablyconsisting of single crystal piezoelectric material) arranged over acavity or recess 82 defined in a slow wave propagation layer orsacrificial material layer underlying the piezoelectric layer 30 todefine a suspended portion 88 of the piezoelectric layer 30. An optionalintermediate layer 27 (which may serve as an etch stop, bonding, orinterface layer) and an underlying substrate 28A are provided below thecavity or recess 82. The cavity or recess 82 serves as a guided waveconfinement structure to confine a lateral acoustic wave in thepiezoelectric layer 30. Internal electrodes (i.e., interdigitaltransducers 58, 58A forming a delay line 84′) are supported by a lowersurface 30″ of the suspended portion 88 of the piezoelectric layer 30. Afirst group of electrodes 38, 40 of opposing polarity form a firstinterdigital transducer 58, and a second group of electrodes 38A, 40A ofopposing polarity form a second interdigital transducer 58A that islaterally spaced from the first interdigital transducer 58. The IDTs 58,58A include bus bars 52 and fingers 54. In certain embodiments, optionalvertical openings 86 (shown in FIG. 21A) may be provided duringmanufacture to enable an etchant to be supplied through thepiezoelectric layer 30 to an underlying layer (sacrificial or slow wavepropagation layer) to define the cavity or recess 82, while theintermediate layer 27 may prevent interaction between etchant and thesubstrate 28A. The functional material and/or sensing material 80 mayinclude any suitable functional material or sensing material describedherein. In certain embodiments, multiple functional material and/orsensing materials 80 may be provided.

In operation of the MEMS guided wave device of FIGS. 21A and 21B,voltage is applied across bus bars 52 of a transmitting IDT 58, and anacoustic wave is propagated through the suspended portion 88 of thepiezoelectric layer 30 (overlaid with the functional material or sensingmaterial 80) to a receiving IDT 58A. Any changes in wave propagationproperties may be used to detect interaction between the functionalmaterial or sensing material 80 and a surrounding environment.

By providing the functional material and/or sensing material 80 over thesuspended portion 88 of the piezoelectric layer 30, greater sensitivityand/or faster response may be obtained. In certain embodiments, thecavity or recess 82 may be sealed. In other embodiments, the cavity orrecess 82 may remain unsealed, to avoid interference between detectionof pressure and detection of other conditions by the functional materialand/or sensing material 80.

FIG. 22A is a top view, and FIG. 22B is a cross-sectional view, of aMEMS guided wave device bearing similarity to the device of FIGS. 20Aand 20B, but with addition of a functional material or sensing material80 over a suspended portion 88 of a piezoelectric layer 30, and omittingreflector gratings. The piezoelectric layer 30 has an exposed uppersurface 30′, with electrodes (including interdigital transducers)embedded in a slow wave propagation layer 26 underlying thepiezoelectric layer 30. A guided wave confinement structure 28 underliesthe slow wave propagation layer 26, with an intermediate layer 27 (whichmay serve as an etch stop, bonding, or interface layer) optionally beingarranged therebetween. As shown in FIG. 20B, the suspended portion 88 ofthe piezoelectric layer 30 is arranged over a cavity or recess 82Adefined in the slow wave propagation layer 26. In certain embodiments,optional vertical openings 86 (shown in FIG. 20A) may be provided toenable an etchant to be supplied through the piezoelectric layer 30 tothe slow wave propagation layer 26 to define the cavity or recess 82A.In alternative embodiments, the cavity or recess 82A may be pre-definedin the slow wave propagation layer 26 (e.g., via etching) beforeattachment of the piezoelectric layer 30 (e.g., via direct bonding) overthe slow wave propagation layer 26. The piezoelectric layer 30 isarranged over electrodes including interdigital transducers 58, 58Aembedded in an upper portion of the slow wave propagation layer 26. TheIDTs 58, 58A include bus bars 52 and fingers 54. In the IDTs 58, 58A,one IDT 58 includes a first group of alternating electrodes 38, 40, andthe other IDT 58A includes a second group of alternating electrodes 38A,40A. The IDTs 58, 58A and piezoelectric layer 30 form a sensor delayline 84′. In operation of the MEMS guided wave device of FIGS. 22A and22B, voltage is applied across bus bars 52 of a transmitting IDT 58, andan acoustic wave is propagated through a delay path including thesuspended portion 88 of the piezoelectric layer 30 (overlaid with thefunctional material or sensing material 80) to a receiving IDT 58A. Anychanges in wave propagation properties may be used to detect interactionbetween the functional material or sensing material 80 and a surroundingenvironment.

In certain embodiments, a MEMS guided wave device including an exposedpiezoelectric layer, electrodes embedded in an underlying layer, andsensing material arranged over at least a portion of the piezoelectriclayer may be incorporated into a microfluidic device. In such a device,liquid may be supplied to and/or over the sensing material.

FIG. 23 is a cross-sectional view of a microfluidic device 98incorporating a MEMS guided wave device including a piezoelectric layer30 having an upper surface 30′ and groups of electrodes 38, 40, 38A, 40Aembedded in a slow wave propagation layer 26 underlying thepiezoelectric layer 30. A guided wave confinement structure 28 isfurther provided below the slow wave propagation layer 26. A sensingmaterial 80 is provided over a portion of the piezoelectric layer 30between a first group of electrodes 38, 40 (preferably configured as afirst IDT) and a second group of electrodes 38A, 40A (preferablyconfigured as a second IDT). Multiple microfluidic structure layers91-93 are stacked over the piezoelectric layer 30 to define at least onemicrofluidic channel 90 and vias 96A, 96B in communication with ports94A, 94B to permit passage of fluid through the microfluidic device 98.The term “microfluidic” as used herein refers to structures or devicesthrough which one or more fluids are capable of being passed or directedand having at least one dimension less than about 500 microns. Incertain embodiments, at least one of the microfluidic structure layers91-93 may include a stencil layer, which refers to a material layer orsheet that is preferably substantially planar through which one or morevariously shaped and oriented portions have been cut or otherwiseremoved (e.g., by laser ablation, blade cutting, etching, or the like)through the entire thickness of the layer, and that permits substantialfluid movement within the layer. The outlines of the cut or otherwiseremoved portions form the lateral boundaries of microstructures that areformed when a stencil is sandwiched between other layers. Microfluidicstructure layers 91-93 may be affixed to one another and to thepiezoelectric layer 30 by any suitable means such as adhesive bonding,direct bonding, clamping, or the like.

In operation of the microfluidic device 98, a fluid is supplied via aninlet port 94A and inlet vias 96A to a microfluidic channel 90 that isbounded in part by the upper surface 30′ of the piezoelectric layer 30,and that contains the sensing material 80. In certain embodiments, thefluid may flow through the microfluidic channel 90 to outlet vias 96Band an outlet port 96B to exit the device 98. While fluid is presentwithin the microfluidic channel 90, an acoustic wave is transduced inthe piezoelectric layer 30 by application of voltage to a transmittingIDT (including electrodes 38, 40), and such wave is propagated through acentral (delay) region of the piezoelectric layer 30 overlaid with thesensing material 80 to be received by a receiving IDT (includingelectrodes 38A, 40A). Interaction between the fluid (or its contents)and the sensing material 80 may alter one or more wave propagationproperties of the piezoelectric layer 30, and may be detected by thereceiving IDT. Although not shown in FIG. 23, a reference delay lineand/or multiple sensing delay lines (as described hereinabove) may beprovided in a single microfluidic device in certain embodiments.

In certain embodiments, at least one functional layer is arranged on orover at least a portion of an exposed piezoelectric layer, andconfigured to interact with the piezoelectric layer to provide mixeddomain signal processing utility. More specifically, in certainembodiments, a micro-electrical-mechanical system (MEMS) mixed domainguided wave device includes: a piezoelectric layer; a plurality ofelectrodes disposed below the piezoelectric layer and configured fortransduction of at least one lateral acoustic wave in the piezoelectriclayer; a guided wave confinement structure configured to confine the atleast one lateral acoustic wave in the piezoelectric layer; and at leastone functional layer arranged on or over at least a portion of thepiezoelectric layer, and configured to interact with the at least onelateral acoustic wave in the piezoelectric layer to provide mixed domainsignal processing utility. Such utility may include, for example,acousto-semiconductor, acousto-magnetic, or acousto-optic signalprocessing utility. In certain embodiments, the at least one functionallayer includes one or more of a conductive material, a semiconductingmaterial, or a dielectric material. In certain embodiments, the at leastone functional layer includes one or more of a piezoelectric material, aferroelectric material, a ferromagnetic material, or a magnetostrictivematerial. In certain embodiments, the at least one functional layerincludes one or more of an optically responsive material, a pyroelectricmaterial, or an organic material. In certain embodiments, the at leastone functional layer in combination with the underlying layers forms anon-reciprocal device in which an input signal transduced by a first IDTis received by a second IDT under specified conditions.

In certain embodiments, the at least one functional layer includes atleast one semiconducting layer, a first electrical contact arranged overa first portion of the at least one semiconducting layer, and a secondelectrical contact arranged over a second portion of the at least onesemiconducting layer; and the at least one semiconducting layer isconfigured to interact with the piezoelectric layer to provide acousticamplification utility.

FIG. 26 illustrates a MEMS mixed domain guided wave device suitable forproviding acoustic amplification utility. The device includes apiezoelectric layer 30 having an exposed upper surface 30′, wherein thepiezoelectric layer 30 overlies alternating electrodes 38, 40 of a firstIDT and alternating electrodes 38A, 40A of a second IDT embedded in anupper portion of an underlying slow wave propagation layer 26 thatoverlies a guided wave confinement structure 28. A portion of thepiezoelectric layer 30 is overlaid with a semiconductor layer 100 (e.g.,GaN, ZnO, or the like), wherein first and second electrical contacts102, 104 are arranged over portions of the semiconductor layer 100. Thesemiconductor layer 100 may be applied over the piezoelectric layer 30by any suitable method, such as direct bonding, layer transfer,epitaxial growth, or other methods known in the art.

In operation of the device shown in FIG. 26, an acoustic wave transducedby at least one group of electrodes 38, 40, 38A, 40A travelling throughthe piezoelectric layer 30 generates an alternating longitudinalelectric field travelling at the acoustic wave velocity. The electricfield, which is non-uniform, induces formation of periodic bunches ofelectric charge throughout the piezoelectric layer 30. Upon applicationof a DC bias voltage across the electrical contacts 102, 104, theelectric charge bunches are augmented by charge carriers in response tothe alternating electric field. The augmented field in turn reacts uponthe piezoelectric layer 30, causing additional acoustic wave components.When the drift velocity of the charge carriers exceeds the velocity ofan acoustic wave in the piezoelectric layer 30, the acoustic wave isamplified. Maximum amplification of the acoustic wave occurs at a steadystate bias voltage that induces a corresponding drift velocity of thecharge carriers.

FIG. 27 illustrates a MEMS mixed domain guided wave device suitable forproviding acoustic amplification utility similar to the device of FIG.26, but including a cavity or recess 82 defined in at least oneunderlying layer to form a suspended portion 88 of an exposedpiezoelectric layer 30. The exposed piezoelectric layer 30 (preferablyconsisting of single crystal piezoelectric material) is arranged overthe cavity or recess 82 defined in at least one layer (e.g., a slow wavepropagation layer or sacrificial material layer) underlying thepiezoelectric layer 30 to define a suspended portion 88 of thepiezoelectric layer 30. An optional intermediate layer 27 (which mayserve as an etch stop, bonding, or interface layer) and an underlyingsubstrate 28A are provided below the cavity or recess 82. The cavity orrecess 82 serves as a guided wave confinement structure to confine alateral acoustic wave in the piezoelectric layer 30. Internal electrodesare supported by a lower surface 30″ of the suspended portion 88 of thepiezoelectric layer 30. A first group of electrodes 38, 40 of opposingpolarity form a first interdigital transducer, and a second group ofelectrodes 38A, 40A of opposing polarity form a second interdigitaltransducer that is laterally spaced from the first interdigitaltransducer. Presence of the cavity or recess 82 alters wave propagationproperties of the piezoelectric layer 30 relative to solid mounting ofthe entire piezoelectric layer 30 to the slow wave propagation layer 26.The suspended portion 88 of the piezoelectric layer 30 is overlaid witha semiconductor layer 100 (e.g., GaN, ZnO, or the like), wherein firstand second electrical contacts 102, 104 are arranged over portions ofthe semiconductor layer 100. The semiconductor layer 100 may be appliedover the piezoelectric layer 30 by any suitable method, such as directbonding, epitaxial growth, or other methods known in the art. Operationof the device of FIG. 27 is similar to that previously described for thedevice of FIG. 26.

In certain embodiments, at least one functional layer of a MEMS mixeddomain guided wave device includes a first semiconducting layer having afirst bandgap and a second semiconducting layer having a second bandgapthat differs from the first bandgap. The first semiconducting layer andthe second semiconducting layer form a heterostructure configured toform a two-dimensional electron gas at an interface between the firstsemiconducting layer and the second semiconducting layer. Examples ofsemiconductor combinations suitable for forming a two-dimensionalelectron gas at an interface therebetween include, but are not limitedto, GaN/AlGaN heterostructures and GaAs/AlGaAs heterostructures.

FIG. 28 illustrates a MEMS mixed domain guided wave device including aheterostructure configured to form a two-dimensional electron gas (2DEG)between two semiconductor layers. The device includes a piezoelectriclayer 30 having an exposed upper surface 30′, wherein the piezoelectriclayer 30 overlies alternating electrodes 38, 40 of a first IDT andalternating electrodes 38A, 40A of a second IDT embedded in an upperportion of an underlying slow wave propagation layer 26 that overlies aguided wave confinement structure 28. A portion of the piezoelectriclayer 30 is overlaid with stacked semiconductor layers 112, 114,preferably with at least one electrode 110 that preferably comprises anohmic contact. In certain embodiments, multiple top side electrodes maybe provided, and/or an additional electrode may be provided inconductive communication with the lower semiconductor layer 114. Forexample, another ohmic contact may be provided to the lowersemiconductor layer 114 at a different cross-section than shown in FIG.28. Formation of a semiconductor heterostructure over a piezoelectrictransducer permits acoustic waves to be launched into a 2DEG, such thateach acoustic wave may serve to transport one or more electrons.Alternatively, or additionally, the 2DEG may be selectively formed toaffect acoustic response. For example, frequency or delay time of theguided wave device may be altered by presence or absence of a 2DEGformed along an interface between the semiconductor layers 112, 114. Incertain embodiments, charge may be selectively injected as an acousticwave travels under the at least one electrode 110. Other utilities maybe provided with a semiconductor-based heterostructure provided over anexposed piezoelectric layer of a MEMS guided wave device, as will berecognized by one skilled in the art. As will also be recognized by oneskilled in the art, in certain embodiments, the semiconductor layers112, 114 may be extended over any suitable area of the piezoelectriclayer 30, such as to overlap the IDTs formed by the electrodes 38, 40,38A, 40A.

In certain embodiments, at least one functional layer formed over anexposed piezoelectric layer surface includes at least one semiconductinglayer; and a source (ohmic) contact, a gate (Schottky) contact, and adrain (ohmic) contact are operatively arranged with the at least onesemiconducting layer to serve as a transistor.

FIG. 29 illustrates a MEMS mixed domain guided wave device in which aportion of a piezoelectric layer 30 is overlaid with at least onesemiconductor layer 118, an oxide layer 120, a source contact 102, agate contact 106, and a drain contact 104 configured to form atransistor. Selective application of voltage to the gate contact 106permits regulation of current flow between the source contact 102 andthe drain contact 106. In certain embodiments, the at least onesemiconductor layer 118 may include multiple semiconductor layers ofdifferent bandgaps providing a heterostructure suitable for forming a2DEG, thereby yielding a high electron mobility transistor. Thepiezoelectric layer 30, which includes an exposed upper surface 30′,overlies alternating electrodes 38, 40 of a first IDT and alternatingelectrodes 38A, 40A of a second IDT embedded in an upper portion of anunderlying slow wave propagation layer 26 that overlies a guided waveconfinement structure 28. In operation of the device, an acoustic wavetransduced by at least one group of electrodes 38, 40, 38A, 40A travelsthrough the piezoelectric layer 30. In certain embodiments, transistoroperation can be modulated by the acoustic wave. Other interactionsbetween an acoustic wave and transistor operation may alter response orother performance characteristics of the transistor and/or of theacoustic resonator.

In certain embodiments, the MEMS mixed domain guided wave device furtherincludes an inner conductive layer arranged within a portion of the slowwave propagation layer, and an outer conductive layer arranged on orover a portion of the piezoelectric layer, wherein the inner conductivelayer and the outer conductive layer are configured to interact with thepiezoelectric layer to provide acoustoelectric convolver utility.

FIG. 30 is a cross-sectional view of a MEMS mixed domain guided wavedevice in which a portion of a piezoelectric layer 30 is utilized aspart of an acoustoelectric convolver that incorporates at least onesemiconductor layer. The piezoelectric layer 30, which includes anexposed surface 30′, overlies alternating electrodes 38, 40 of a firstIDT and alternating electrodes 38A, 40A of a second IDT embedded in anupper portion of an underlying slow wave propagation layer 26, whichoverlies a guided wave confinement structure 28. A central portion ofthe slow wave propagation layer 26 (between groups of alternatingelectrodes 38, 40 and 38A, 40A) includes an additional (output)electrode in the form of an embedded plate electrode 130, which isarranged in contact with a lower surface of the piezoelectric layer 30.A central portion of the piezoelectric layer 30 is overlaid with stackedfunctional layers, including a dielectric layer 126, a semiconductorlayer 124, and a top side plate electrode 122. A sandwich structureincluding the central portion of the piezoelectric layer 30, thedielectric layer 126, and the semiconductor layer 124 arranged betweenthe top side plate electrode 122 and the embedded plate electrode 130provides a parallel plate acousto-electric convolver structure.

In operation of the device of FIG. 30, a coded input signal having afrequency f is supplied to alternating electrodes 38, 40 of a first IDT,and a time-reversed reference code having a frequency f is supplied toalternating electrodes 38A, 40A of a second IDT. Acoustic wavesgenerated in the piezoelectric layer 30 are received by the sandwichstructure (including the central portion of the piezoelectric layer 30,the dielectric layer 126, and the semiconductor layer 124 arrangedbetween the top side plate electrode 122 and the embedded plateelectrode 130), providing an autocorrelated output signal at a frequencyof 2f through an output electrode comprising in combination theelectrodes 122, 130.

Alternative parallel plate acousto-electric convolver structures may beprovided. In certain embodiments, a MEMS mixed domain guided wave devicefurther includes at least one semiconducting layer arranged between theinner conductive layer and the outer conductive layer. FIG. 31 is across-sectional view of a MEMS mixed domain guided wave device similarto the device of FIG. 30, but with two dielectric layers 126, 128accompanying the semiconductor layer 124 within a sandwich structure ofan acousto-electric convolver. The piezoelectric layer 30, whichincludes an exposed upper surface 30′, overlies alternating electrodes38, 40 of a first IDT and alternating electrodes 38A, 40A of a secondIDT embedded in an upper portion of an underlying slow wave propagationlayer 26, which overlies a guided wave confinement structure 28. Acentral portion of the slow wave propagation layer 26 (between groups ofalternating electrodes 38, 40 and 38A, 40A) includes an additionalelectrode in the form of an embedded plate electrode 130, which isarranged in contact with a lower surface of the piezoelectric layer 30.A central portion of the piezoelectric layer 30 is overlaid with stackedfunctional layers, including the first dielectric layer 126, thesemiconductor layer 124, the second dielectric layer 128, and a top sideplate electrode 122. A sandwich structure including the central portionof the piezoelectric layer 30, the dielectric layers 126, 128, and thesemiconductor layer 124 arranged between the top side plate electrode122 and the embedded plate electrode 130 provides a parallel plateacousto-electric convolver structure. Operation of the device of FIG. 31is similar to that previously described for the device of FIG. 30.

In certain embodiments, at least one functional layer of a mixed domainguided wave device includes at least one semiconducting layer; theplurality of electrodes includes a first group of input electrodesconfigured for transduction of a first lateral acoustic wave in thepiezoelectric layer and a second group of input electrodes configuredfor transduction of a second lateral acoustic wave in the piezoelectriclayer; output electrodes of opposing polarity are provided in ohmiccontact with the at least one semiconducting layer; and the first groupof input electrodes, the second group of input electrodes, and theoutput electrodes are configured to interact with the piezoelectriclayer to provide acoustic wave convolver with bidirectionalamplification utility.

One example of a mixed domain guided wave device configured to provideacoustic wave convolver with bidirectional amplification utility isshown in FIG. 32. Such device includes a piezoelectric layer 30 havingan exposed upper surface 30′, wherein the piezoelectric layer 30overlies alternating electrodes 38, 40 of a first IDT and alternatingelectrodes 38A, 40A of a second IDT embedded in an upper portion of anunderlying slow wave propagation layer 26 that overlies a guided waveconfinement structure 28. A portion of the piezoelectric layer 30between the respective IDTs is overlaid with at least one semiconductorlayer 132 that is further overlaid with ohmic contacts 134, 136 ofopposing polarity that yield an output IDT. In certain embodiments, asingle semiconductor layer 132 may be provided, but in alternativeembodiments, multiple semiconductor layers 132 may be provided, and mayoptionally form a heterostructure with a significant bandgap differencebetween layers. In the mixed domain device, convolution may occurprimarily in the semiconductor layer 132, but may be further augmentedby the piezoelectric layer 30. As will be recognized by one skilled inthe art, the ohmic contacts 134, 136 of the output IDT may differsubstantially from the contacts 38, 40, 38A, 40A of the first and secondIDTs with respect to material, number, periodicity, and/or otherproperties. In operation of the device of FIG. 32, signals of the sameor different frequencies may be supplied through the first IDT(including alternating electrodes 38, 40) and through the second IDT(including alternating electrodes 38A, 40A), and a DC voltage may beapplied across the ohmic contacts 134, 136 of the output IDT. Acousticwaves transduced by the first and second IDTs are propagated through thepiezoelectric layer 30 to the at least one semiconductor layer 132 andreceived by the ohmic contacts 134, 136 of the output IDT. Interactionbetween the acoustic waves and the at least one semiconductor layer 132,as well as the piezoelectric layer 30, effects convolution of the inputsignals, and results in generation of an output signal detected by theohmic contacts 134, 136 of the output IDT.

Embodiments as disclosed herein may provide one or more of the followingbeneficial technical effects: enablement of adjustment of one or moreproperties of a guided wave device; integration of one or morefunctional and/or sensing structures with a guided wave device withoutinterfering with placement of acoustic electrodes; providing guided wavedevices with enhanced utility; and facilitating efficient manufacture ofguided wave devices, including those with enhanced utility.

Upon reading the following description in light of the accompanyingdrawing figures, those skilled in the art will understand the conceptsof the disclosure and will recognize applications of these concepts notparticularly addressed herein. Those skilled in the art will recognizeimprovements and modifications to the preferred embodiments of thepresent disclosure. All such improvements and modifications areconsidered within the scope of the concepts disclosed herein and theclaims that follow. Any of the various features and elements asdisclosed herein may be combined with one or more other disclosedfeatures and elements unless indicated to the contrary herein.

What is claimed is:
 1. A micro-electrical-mechanical system (MEMS)guided wave device comprising: a piezoelectric layer; a plurality ofelectrodes arranged in a slow wave propagation layer disposed below thepiezoelectric layer and configured for transduction of a lateralacoustic wave in the piezoelectric layer; and a guided wave confinementstructure arranged proximate to the slow wave propagation layer andconfigured to confine the lateral acoustic wave in the piezoelectriclayer and the slow wave propagation layer; wherein the piezoelectriclayer comprises a first thickness region and a second thickness region,and a thickness of the first thickness region differs from a thicknessof the second thickness region.
 2. The MEMS guided wave device of claim1, wherein the piezoelectric layer comprises a single crystalpiezoelectric material.
 3. The MEMS guided wave device of claim 1,wherein the plurality of electrodes comprises a first plurality ofelectrodes arranged against or adjacent to the first thickness regionand configured for transduction of a first lateral acoustic wave havinga wavelength λ₁ in the first thickness region, and comprises a secondplurality of electrodes arranged against or adjacent to the secondthickness region and configured for transduction of a second lateralacoustic wave having a wavelength λ₂ in the second thickness region,wherein λ₂ differs from λ₁.
 4. The MEMS guided wave device of claim 3,wherein the first plurality of electrodes includes a first interdigitaltransducer (IDT) comprising a first two groups of electrodes of opposingpolarity and comprising a first spacing between adjacent electrodes ofopposing polarity of the first two groups of electrodes of opposingpolarity; the second plurality of electrodes includes a secondinterdigital transducer (IDT) comprising a second two groups ofelectrodes of opposing polarity and comprising a second spacing betweenadjacent electrodes of opposing polarity of the second two groups ofelectrodes of opposing polarity; and the second spacing differs from thefirst spacing.
 5. The MEMS guided wave device of claim 1, wherein theguided wave confinement structure comprises a fast wave propagationlayer or a Bragg mirror.
 6. The MEMS guided wave device of claim 1,wherein the plurality of electrodes is arranged in contact with thepiezoelectric layer.
 7. The MEMS guided wave device of claim 2, furthercomprising a bonded interface between the piezoelectric layer and atleast one underlying layer of the MEMS guided wave device.
 8. The MEMSguided wave device of claim 1, further comprising a substrate underlyingthe guided wave confinement structure.
 9. The MEMS guided wave device ofclaim 1, further comprising at least one functional layer at leastpartially covering the piezoelectric layer.
 10. The MEMS guided wavedevice of claim 1, further comprising a first functional layer at leastpartially covering the first thickness region, and a second functionallayer at least partially covering the second thickness region, whereinthe second functional layer differs from the first functional layer inat least one of material composition, thickness, or materialconcentration.
 11. The MEMS guided wave device of claim 1, furthercomprising at least one loading material arranged on or proximate to thepiezoelectric layer and configured to locally alter a property of thelateral acoustic wave in the piezoelectric layer.
 12. A method offabricating a MEMS guided wave device, the method comprising: defining aplurality of electrodes on a piezoelectric layer; depositing a slow wavepropagation layer over the plurality of electrodes and at least aportion of the piezoelectric layer; providing a guided wave confinementstructure on or adjacent to the slow wave propagation layer, wherein theguided wave confinement structure is configured to confine a lateralacoustic wave in the piezoelectric layer and the slow wave propagationlayer; and locally thinning the piezoelectric layer to define a firstthickness region and a second thickness region, wherein a thickness ofthe first thickness region differs from a thickness of the secondthickness region.
 13. The method of claim 12, wherein the piezoelectriclayer comprises a single crystal piezoelectric material.
 14. The methodof claim 12, further comprising planarizing a surface of the slow wavepropagation layer prior to said providing of the guided wave confinementstructure on or adjacent to the slow wave propagation layer.
 15. Themethod of claim 12, wherein said local thinning of the piezoelectriclayer comprises etching.
 16. The method of claim 12, further comprisingdepositing at least one of a functional material or a loading materialat least partially covering the piezoelectric layer after said localthinning of the piezoelectric layer.
 17. A method of fabricating a MEMSguided wave device, the method comprising: defining a plurality ofelectrodes in a slow wave propagation layer; bonding or depositing apiezoelectric layer on or over the slow wave propagation layer; andlocally thinning the piezoelectric layer to define a first thicknessregion and a second thickness region, wherein a thickness of the firstthickness region differs from a thickness of the second thicknessregion.
 18. The method of claim 17, wherein the piezoelectric layercomprises a single crystal piezoelectric material.
 19. The method ofclaim 17, wherein said defining of the plurality of electrodes in theslow wave propagation layer comprises: defining a plurality of recessesin the slow wave propagation layer; and depositing electrode material inthe plurality of recesses.
 20. The method of claim 19, furthercomprising planarizing a surface of the slow wave propagation layerprior to said bonding or depositing of the piezoelectric layer on orover the slow wave propagation layer.